Originally Posted by SRDYou can use a disarm action to snatch an item worn by the target. If you want to have the item in your hand, the disarm must be made as an unarmed attack. (...) This otherwise functions identically to a disarm attempt, as noted above.
Originally Posted by SRDUnarmed Attacks
(...) Attacking unarmed provokes an attack of opportunity from the character you attack, provided she is armed. (...)
An unarmed character can’t take attacks of opportunity (but see "Armed" Unarmed Attacks, below).
"Armed" Unarmed Attacks
Sometimes a character’s or creature’s unarmed attack counts as an armed attack. A monk, a character with the Improved Unarmed Strike feat, a spellcaster delivering a touch attack spell, and a creature with natural physical weapons all count as being armed.
Note that being armed counts for both offense and defense (the character can make attacks of opportunity)