Well they are called Opportunity Attacks (OA) now and yes, anytime you "leave" a threatened square you involve one. Just moving around someone does not. BUT anyone can try an Dexterity (Acrobatics) check to move by someone without provoking a OA if you meet their Reflex defense, and if you make it by Reflex +5 you can pass through someone else's square (normally you can't move through a square where someone else is standing, this allows you to do that and not take an OA). Passing through an enemy space in this way also is considered Difficult Terrain (meaning you would have to use 4 squares of movement to pass through someone of Medium size's space)