Ok, house rulings here....
Yes, you can. Anyone can drop thier gaurd momentarily and allow an opponent an opening to attack them.
However, if you are doing so for a devious reason, your opponent gets a Sense Motive check (DC 15) to discern your evil plan.
As a standard action (rather than a free action), you can provoke an attack of oppurtunity which you are actually in fact secretly prepared for and recieve a +2 circumstance bonus on your AC versus that attack. Again, your opponent gets a sense motive check in order to catch on to your devious ways. You may oppose his check with your bluff check.
If the character has fallen for this plan before, and suffered the consequences, he'll recieve a circumstance bonus on his Sense Motive check. Once bitten, twice shy.