"Lightning Grab.
Prerequisites: Improved Initiative, Dex 15+
Benefit: You may take an item from any flatfooted opponent within 5 feet with a successful pick pocket check opposed by your opponents reflex save. You may take any item that you could normally carry, and may grab anything you can see on your opponent. For example, you coud take a dagger from an orc's belt sheath, but you could not take it if he had it out of sight in his belt pouch. Any item that could be freely picked up, without undoing bluckes or knots, is vulnerable to this feat."
That sound more like a manuever to me.
It would take a pretty hidebound DM to not allow this without the feat (for instance what is the 'normal' rule for handling this situation?), and a fairly ungenerous DM to not allow a pickpocket to use his skill rather than an attack roll. In theory, that's what pickpocket is for anyway, and all we've changed here is that he doesn't care if he's caught (which ought to make the roll that much easier).
a) A person who is flat footed has no threat zone, and thus cannot make a AoA
b) If you were attempting to disarm an opponent, then he could resist with an opposed roll. At the least, you could rule that taking a dagger from an opponent's sheath was a disarm. If you make an unarmed disarm, you are now holding the weapon (ei you grabbed it).
c) However, it seems reasonable that grabbing something that your opponent is not holding is easier than grabbing something that he is. I would rule that the only thing needed is a touch attack modified by the items size and possibly cover (if it is in the opinion of the DM behind the target relative to you). Since the target is both flat footed and not holding the item, I don't see why he gets an opposed roll.
d) Whether you wish to make a touch attack or a pick pocket check vs. DC 20, you still grab something successfully - feat or not.
Now if the feat let you do this without provoking an attack of opportunity, that might be more like a feat.
In this particular case, I'd say this was a unarmed disarm. Of course, you could grapple first and then attempt to wrestle it off (hopefully under more favorable circumstances), but eventually a disarm check is required. If you wanted to get particular, some helmets are probably like locked gauntlets (have straps, form fitting, etc.) and thus have a +10 bonus to staying on. On the other hand, one could rule that the helmet wearer had a non-proficiency penalty (-4) to resist since it is not a normal weapon and is not held like one.