It was speculated this would happen, but it hasn't been made official until now. Sony has announced that Warhawk will be abandoning single-player in favor of a multiplayer-only battle arena. Therefore, it will only be available on the Network, but the game should also sport a couple new features designed to emphasize the game's new focus. It's not just about flying; players will also commandeer land vehicles like jeeps, tanks, and even the very fragile infantry units.
And there will be more than a couple online modes, too, so don't think for a second that multiplayer-only is a major limitation. We'll be seeing Warhawk on the PlayStation Store some time this fall.
Related Game(s): Warhawk