Slip Past: Part of a move Action
✦ Opposed Check: Acrobatics vs. Reflex
✦ Success: Treat the target's space as difficult terrain and move though (provoking Opportunity Attacks as normal).
✦ Failure: Fall prone in the square prior to entering the target's space; the target can make a melee basic attack against you as a free action and has combat advantage for the attack.
Of course, if you rule that PCs can use these actions in combat, it also means that monsters can use them to their advantage as well...