"EA have really got behind the whole "free to play" thing of late, announcing it'll form the basis of future Tiger Woods and Need for Speed titles. The first such game to test those waters, however, is Battlefield Heroes.
The premise is simple: take Battlefield, then make it absolutely free to play, by stripping some elements out and reserving them for paying customers, while supporting the whole thing via front-end advertising.
Such a move is like walking a knife's edge. Make too much free and there's no point running it as a product. Make too much paid content and you'll piss people off who want to play for free."