First off,
Just a quick question, by the logic of some people, you cannot use a trip attempt with the weapon,
unless it is in the weapons text. Now if you look at the following weapons;It does look as if you could only trip using them because it states they may be used for a trip attempt.
Chained Spike: Because the chain can wrap around an enemy's leg or other limbyou can make trip attacks. If you are being tripped during your attempt, you can avoidbeing tripped. PG 99
Flail heavy or Light: You can also use this weapon to make trip attacks. If you are tripped during your attempt, you can drop the flail to avoid being tripped. PG 100
Guisarme: Because of a Guisarme's curved blade, you can also make trip attacks. If you are tripped during your attempt, you can drop guisarme to avoid beign tripped. PG 101
Halberd: Because of the hookon the back of the halberd you can use it to make trip attacks. if you are tripped during your own trip attempt you can drop the halberd to avoid being tripped. PG 101
Whip: Becuase the whip can wrap around an enemey's leg or other limb, you can make trip attacks with it. If you are tripped during your own trip attempt, you can drop the whipto avoid being tripped. PG 104
From the description it does say that these weapons can be used to make trip atatcks, the reason though that it makes special references to these weapons is because of the design of them. They have a special advantge over other weapons in a trip because of a hook or rapping affect, the character does not need to lunge as close into an aattempt as with a normal weapon. This is why he can drop the weapon to avoid the trip, because he doesn't get as close as with a standard weapon. making sense?
Secondly;
AGGEMAM said:
Some weapons are clearly noted to be trip weapon in that thier description says that can used to trip, these weapon even have special ability in regards to tripping as being able to drop the weapon to avoid a counter-trip.
Obviously that means that all trip performed without such weapons are unarmed and thereby do provoke AoOs if you are not considered armed with your unarmed attacks.
Did you read improved trip, information in the PHB on page 139. No where in there does it say that you provoke attacks of opportunity, also your logic that all trips are unarmed if flawedin my oppinion (not intending offence with this post). Acording to your argument unless the weapon states within its descritpion that it can make a trip attempt, you cannot. Unarmed Strike is considered a weapon. No were in its text does it reference to trip atatcks.
Unarmed Strike A medium-size haracter deals 1d3 points of subdual damage with an unarmed strike, which may be a punch kick, head butt, or other type of attack. A small character deals 1d2 points of subdual damage. the damage from an unarmed strike is considered weapon damage for the purposes of effects that give you a bonus to weapon damage.
You can use the Weapon Finese feat (see page 86) to applu your dexterity modifer instead of yoru strength modifier to attack roll with an unarmed strike. PG 103
No where in the weapon description does it say you can make a trip attack, to under the explanation you provide, unarmed does not work either. How does this make sense?
I say that these weaposn with special reference to trips is simply because they have a special circumstance, not that only these weapons may be used to trip atatcks