Meh, for those of us who aren't extremely familiar with ancient weapons, it doesn't matter that D&D's "falchion" and "longsword" and "short sword" don't really go by the names they would've historically, it's just a game. The falchion could be called a great scimitar or whatever, the long sword a war sword, and the short sword a gladius.
Anyway..... I have no problem with the first poster's version of Katana stats. Katanas were made from poor steel and needed the folding techniques to work out impurities, but they were strong for cutting, not as much for impacts or thrusting though. European swords were tougher perhaps, and better for thrusting IIRC, but the katana had its advantages when cutting lightly-armored foes (like those in leather or lacquered leather armor). I don't agree though with some other posters that 'a medium sword is a medium sword is a medium sword', or in other words, 'all swords of similar length and mass are the same'. Also, I think it's fine to tweak things for D&D; it's a fantasy game after all, not a full-bore simulation of historical combat.
Here's my version of the Katana for D&D, in a spoiler block, for reference. Also the Wakizashi, Nodachi, and Zanbattou (the latter influenced as much by Sanouske of Rurouni Kenshin as much as by reading a bit about the historical Japanese cavalry-killer sword). Note that these use 3.0 weapon size rules, so 3.5ers might need to check the 3.0 SRD for reference. All of these are exotic melee weapons.
[sblock]Wakizashi.....25 gp.....2 lbs......AC 6.....Hardness 7.....HP 3.....Break DC 14
..........2d3 (Slashing) or 1d4 (Piercing).....T/C 18-20/x2 (19-20/x2)
..........*Each time the wakizashi strikes an enemy with total armor, shield, and natural armor bonuses of +5 or greater, or if it misses an enemy because of armor, shield, and natural armor AC bonuses that are due to force effects, metal shields, or armor made with more metal than scalemail, reduce the wakizashi's Hardness by 1. The wakizashi deals half damage, rounded up, against opponents with total armor, shield, and natural armor bonuses of +5 or greater.
..........*Likewise, reduce wakizashi Hardness by 1 if striking an object, unless the object has Hardness 5 or less, or unless the strike is to parry a weapon or projectile no more than one size category larger than the wakizashi.
..........*If the Hardness is reduced to zero, further reductions are applied to hit points instead, until repaired. These reductions in wakizashi Hardness and hit points last until it is repaired.
..........*A wakizashi can be used as a martial melee weapon if held two-handed by a character of small or medium size, or one-handed by a larger character. A tiny character requires the appropriate Exotic Weapon Proficiency in order to wield a wakizashi one-handed. Size limitations to wielding a weapon one-handed still apply.
..........*Masterwork wakizashi, if wielded proficiently, grant the added benefit of a +2 circumstance bonus on threat confirmation rolls, retained even if magically enhanced.
Katana.....50 gp.....3.5 lbs......AC 5.....Hardness 7.....HP 4.....Break DC 14
..........2d4 (Slashing) or 1d4 (Piercing).....T/C 18-20/x3 (19-20/x2)
..........*Each time the katana strikes an enemy with total armor, shield, and natural armor bonuses of +5 or greater, or if it misses an enemy because of armor, shield, and natural armor AC bonuses that are due to force effects, metal shields, or armor made with more metal than scalemail, reduce the katana's Hardness by 1. The katana deals half damage, rounded up, against opponents with total armor, shield, and natural armor bonuses of +5 or greater.
..........*Likewise, reduce katana Hardness by 1 if striking an object, unless the object has Hardness 5 or less, or unless the strike is to parry a weapon or projectile no more than one size category larger than the katana.
..........*If the Hardness is reduced to zero, further reductions are applied to hit points instead, until repaired. These reductions in katana Hardness and hit points last until it is repaired.
..........*A katana can be used as a martial melee weapon if held two-handed by a character of medium size, or one-handed by a larger character. To wield the katana in one hand requires Strength of at least 13, and this is a prerequisite for gaining any one-handed katana proficiency. A small character requires the appropriate Exotic Weapon Proficiency in order to wield a katana two-handed. Size limitations to wielding a weapon one-handed still apply.
..........*Masterwork katana, if wielded proficiently, grant the added benefit of a +2 circumstance bonus on threat confirmation rolls, retained even if magically enhanced.
No-Dachi (Nodatchi).....65 gp.....10 lbs......AC 4.....Hardness 10.....HP 10.....Break DC 17
..........3d4 (Slashing) or 1d6 (Piercing).....T/C 17-20/x2 (19-20/x2)
..........*Each time the no-dachi strikes an enemy with total armor, shield, and natural armor bonuses of +5 or greater, or if it misses an enemy because of armor, shield, and natural armor AC bonuses that are due to force effects, metal shields, or armor made with more metal than scalemail, reduce the no-dachi's Hardness by 1. The no-dachi deals half damage, rounded up, against opponents with total armor, shield, and natural armor bonuses of +5 or greater.
..........*Likewise, reduce no-dachi Hardness by 1 if striking an object, unless the object has Hardness 5 or less, or unless the strike is to parry a weapon or projectile no larger than the no-dachi.
..........*If the Hardness is reduced to zero, further reductions are applied to hit points instead, until repaired. These reductions in no-dachi Hardness and hit points last until it is repaired.
..........*A no-dachi can be used as a martial melee weapon if held two-handed by a character of large size, or one-handed by a larger character. To wield the no-dachi two-handed requires Strength of at least 15, and this is a prerequisite for gaining any two-handed no-dachi proficiency, while a Strength of 19 is required if the no-dachi is to be wielded one-handed, and likewise any one-handed no-dachi proficiency has a Strength prerequisite of 19+. Size limitations to wielding a weapon one-handed still apply.
..........*The no-dachi's great size incurs a -2 circumstance penalty on all attack rolls with it unless the user is at least large-sized, regardless of proficiency, and this stacks with any nonproficiency penalties when applicable. Likewise, unless the user is at least large-sized or has Strength of at least 19, they cannot perform a full-attack action with the no-dachi and cannot threaten an area with it.
..........*A no-dachi has reach, and can be used to strike an opponent up to 10 feet away. However, the no-dachi deals only 1d6 slashing damage to opponents within 5 feet, cannot be used for piercing damage against opponents within 5 feet, incurs a -2 attack penalty against opponents within 5 feet, and has its threat range reduced by half against opponents within 5 feet.
..........*Any proficient character wielding the no-dachi may use it to perform a Whirlwind Attack, as per the feat, but without need for that feat, and striking all opponents beyond 5 feet who are within the no-dachi's reach. Masterwork no-dachi, if wielded proficiently, grant the added benefit of a +2 circumstance bonus on threat confirmation rolls, retained even if magically enhanced.
Zanmato (Zanbattou)90 gp.....14 lbs......AC 4.....Hardness 10.....HP 13.....Break DC 20
..........4d4 (Slashing) or 1d8 (Piercing).....T/C 20/x3 (19-20/x2)
..........*A zanmato can be used as a martial melee weapon if held two-handed by a character of large size, or one-handed by a larger character. To wield the zanmato two-handed requires Strength of at least 15, and this is a prerequisite for gaining any two-handed zanmato proficiency, while a Strength of 19 is required if the zanmato is to be wielded one-handed, and likewise any one-handed zanmato proficiency has a Strength prerequisite of 19+. Size limitations to wielding a weapon one-handed still apply.
..........*The zanmato's great size incurs a -3 circumstance penalty on all attack rolls with it unless the user is at least large-sized, regardless of proficiency, and this stacks with any nonproficiency penalties when applicable. Likewise, unless the user is at least large-sized or has Strength of at least 19, they cannot perform a full-attack action with the zanmato and cannot threaten an area with it.
..........*A zanmato has reach, and can be used to strike an opponent up to 10 feet away. However, the zanmato deals only 1d6 slashing damage to opponents within 5 feet, cannot be used for piercing damage against opponents within 5 feet, incurs a -3 attack penalty against opponents within 5 feet, and has its threat range reduced by half against opponents within 5 feet.
..........*Any proficient character wielding the zanmato may use it to perform a Whirlwind Attack, as per the feat, but without need for that feat, and striking all opponents beyond 5 feet who are within the zanmato's reach.[/sblock]