Sadly, there's no word yet on what happens once you start playing for free, like how much it'll cost you to get the decent weapons or equipment. Balancing the free players with the paying userbase is always the knife's-edge decision that makes or breaks these kinds of games, and since it broke Battlefield Heroes, here's hoping EA gets it right with this one.
Battlefield Play4Free combines elements of some of the more recent games in the series, like maps from Battlefield 2 and classes and weapons from Battlefield: Bad Company 2. The game launches in Spring 2011. You can sign up for its closed beta here.