Zaruthustran
The tingling means it’s working!
Keith said:I can’t find the reference for not threatening adjacent squares with ranged/projectile weapons. I’m sure it is there, but it is not easy to find in the SRD, at least for me.
To bring the sling into it, nothing prevents a loaded sling from being used as a melee rather than range weapon. Again, seems threatening to me.
Your "use a longbow as a club" and "use a loaded sling as a sap" ideas are neat. But they're not in the rules, and this is the rules forum.

Threatened Squares: You threaten all squares into which you can make a melee attack, even when it is not your action. Generally, that means everything in all squares adjacent to your space (including diagonally). An enemy that takes certain actions while in a threatened square provokes an attack of opportunity from you. If you’re unarmed, you don’t normally threaten any squares and thus can’t make attacks of opportunity.
do people interpret “can make a melee attack” as the character brandishing a melee weapon?
“can make a melee attack” simply means that the character, at the time of the action that caused the AoO, can make a melee attack into the square in question.
That's pretty clear.
Examples:
If you're a Medium size creature with a longsword you threaten all adjacent squares because you can make a melee attack into all adjacent squares.
If you're a Medium size creature wielding a longbow and wearing armor with armor spikes (not spiked gauntlets) you threaten all adjacent squares because you can make a melee attack with armor spikes.
If you're a Medium size creature wielding a longbow you do not threaten any adjacent squares because you cannot make a melee attack with a longbow.
I guess you could argue with your DM that the thin, flexible, light wood of a bow could be used as a weapon (maybe for 1 point of damage and no Str bonus?). That's a judgement call for the DM to make, under the Improvised Weapons rule on page 113. I think you'd have better luck using a crossbow as a club; a bow is too thin to be an effective melee bludgeoning weapon, and if you stabbed someone with it it'd just bend--that's what bows are designed to do, after all. But the rules leave improvised weapons up to the individual DM.
But do note that using a longbow as a melee weapon means that you're NOT using it as a ranged weapon. You'd have to change your grip at the very least. And that should take a free action at the least. Alas, the rules aren't explicitly there.
Or would it be enough to be armed and evidently hostile? Given that an opponent might conceivable draw a weapon and strike at any moment. Or is that “unarmed” as well?
Being hostile is not enough. If you are not armed (and you don't have Improved Unarmed Strike) then you don't threaten. Quick Draw doesn't help at all. Here's a test: ask the question "Can the character, at the time of the action that potentially provokes the AoO, make a melee attack into the square in question?" If the answer is "yes" then you threaten. If the answer is not "yes" then you do not threaten. Note that "Well, if he drew a weapon with Quick Draw..." is not "yes."
"Armed" is clearly not the same as "able to become armed very quickly."
Example:
DM: The orc runs between the two of you, potentially drawing an AoO.
Player1: I have my sword out, so I take that AoO.
DM: Okay. Player2?
Player2: My sword is in my sheath. So first I take a Free Action to Quick Draw my sword, then--
DM: Nope. There's no "first" or "then." Can you make a melee attack--right now--without first taking some other sort of action?
Player2: Er, no.
DM: Well, then no AoO for you.
Why not allow free action as part of an AoO? The rules simply don't allow for it. And, what's more, it opens up for abuse. Here's another example:
DM: The orc runs between the two of you. AoOs?
Player1: I have my sword out, so I take my AoO. And as a free action I cast a quickened Prayer spell.
Player2: I don't have my sword out. So I take a free action to quickdraw my greatsword, swing at him--
Player1: Don't forget the bonus from my Prayer!
Player2: Oh yeah, I swing with the bonus from the quickened Prayer, then after I swing I take a free action to drop the sword, then as a free action I quickdraw my longspear from my Quiver of Ehlonna so that he'll provoke another AoO as he continues past me.
DM: You guys suck.
-do characters wielding bows threaten adjacent squares with that weapon (for the purpose of taking an AOO)?
Let's find out. The SRD says "You threaten all squares into which you can make a melee attack." On page 112 of the PHB it says "Ranged weapons are thrown weapons or projectile weapons that are not effective in melee." A bow is a projectile weapon. A bow is not effective in melee. You cannot make a melee atack with a bow. So, you do not threaten adjacent squares--for the purpose of taking an AOO, or for any other purpose.
If not, does holding a bow preclude the possibility of making a melee attack and hence threatening a square, for the purpose of providing flanking, for example?
No, holding a bow does not preclude the possibility of making a melee attack and hence threatening a square. While it's true that "you need at least two hands to use a bow, regardless of its size", you can threaten with other weapons that are not worn or carried in your hands. Example: armor spikes (knee spike, foot spike, shoulder spike, whatever) or unarmed strike (head butt, kick, knee, whatever).
Hope that helps!
-z