There will be a demo for Battlefield 3 available before the game. Dice's Patrik Bach has told msxbox-world.com. He also took the time to say that the reason for the 24 players in the console versions' multiplayer (compared to the 64 on PC) is simply because it gives the best balance to the whole experience.
Battlefield 3 is slated for release for PC, Playstation 3 and Xbox 360 at some point later this year.