While the makers plan on tightly integrating many types of gameplay between the Xbox 360 and the PC, player-versus-player combat will never be cross-platform. Instanced battles will only take place between players on the same platform to avoid nightmarish balancing issues. There will be cross-play within vehicles (PC gunners can ride shotgun with 360 drivers, for example), and Player-vs-Environment missions will be freely mixed across both as well.
While the game's early good looks are promising, getting our hands on it is a long ways off. The development team told us that they won't have the full game running until next year, so it'll be a long while until this one's in our crosshairs.