Note - A gentleman named Kevin Elmore sent these questions to the Sage and posted his response on the Infinite Monkeys Yahoo group at: http://groups.yahoo.com/group/Infinite_Monkeys/
My comments are in italics.
Question: I have run into some confusion about whether an
unarmed person (or indeed a person holding a bow or a
lantern) can make an attack of opportunity even if he does
not have the Improved Unarmed Attack feat.
Skip's answer: He can't.
Question: According to the PHB, you make an AoO against
someone who performs one of a list of given actions within
one of your threatened squares. A threatened square is a
square into which you can make a melee attack. From the
strictest interpretations, this leads me to believe that
someone who runs past an unarmed wizard does indeed provoke
an attack of opportunity.
Skip's answer: Yes, but the PH is incorrect--you have to
be armed to threaten an area.
Caliban: Unfortunately, Skip cannot rewrite the PHB on his own authority. The Core Rules trump Skip's opinion.
Question: Yet, I have received a lot of resistance from
other players. They claim that the person must have a melee
weapon in his hands in order to take advantage of an attack
of opportunity. So far, I have not seen any rule to support
this, but that is because I refuse to drag the game to a
screeching halt just to look up a rule.
Skip's answer: See the D&D FAQ (available off the D&D page
on the WotC web site).
Caliban: The D&D FAQ doesn't address this issue at all, other than a ruling that confirms that you don't threaten area when wielding a ranged weapon.
Question: Since I seem to be outnumbered in this
interpretation, I decided to come to you. If I am wrong,
could you show me where I have made my mistake? I've pored
over the PHB for quite some time now. I'd be quite happy to
admit my mistake if I can just see the reasoning.
Question: Furthermore, assuming I'm right (and if you show
me to be wrong, then ignore the rest of this message if you
like), what is the result of an unarmed attack of
opportunity against an armed opponent? For example, Bob the
Wizard is trying to stop the evil cultist from running past
him. The cultist has a wicked falchion, while Bob has hands
and feet. Bob makes his attack of opportunity (if my
reasoning is right). Does the cultist get an AoO himself
against Bob since he is armed?
Skip's answer: This is only one of many reasons why you
have to be armed to threaten an area.
Caliban: Unfortunately, Skip doesn't really answer the question. This is easily answered by the core rules: Yes, the cultis get's an AoO in return, if you don't have Improved Unarmed Strike.
Question: I really thought I read somewhere that you can
only make an AoO when it's not your turn.
Skip's answer: That is the definition of an Attack or
Opportunity.
Caliban: That is not the definition of an AoO. The definition of an AoO is on page 275 of the PHB, and doesn't mention anything about only being able to make an AoO when it's not your turn.
Question: But I can't find that in the rulebooks. From the
strictest of rules interpretations, it would seem to me
that an unarmed attack of opportunity can provoke an attack
of opportunity from the defender. This seems odd to me, but
it does highlight the importance of having a weapon.
Skip's answer: Indeed.
Caliban: Yes, it does. It would be better to do a Trip instead of an Unarmed Attack. Trip attacks do not require a weapon, and do not provoke an AoO. (If you use Skips ruling, a person wielding a dagger can trip someone on an AoO, but an unarmed man cannot.)
Question: Can you find any place in the book that
determines one way or the other whether an unarmed AoO
provokes an AoO itself (naturally, assuming no Improved
Unarmed Strike)? I'd really like to clear this up. I can
get around this in a home game with house rules, but I play
Living Greyhawk, which, as you've likely heard, uses the
strictest interpretations of the rules. And I'm not too
proud to know when I need to run to someone else for help
in picking apart rules.
Skip's answer: See the FAQ.
Caliban: The FAQ doesn't mention anything about this.