When Valve announced last year that they were working on the PlayStation 3 version of Portal 2 themselves, and would make it the "best version on any console" complete with features from their Steam PC platform, the true nature of those features were still cloudy. Today, mere days away from the release of Portal 2, Valve has sent out a press release explaining what "Steam on PS3" really means.
Apparently, it means a lot. When you play Portal 2 on PS3, a Steam "overlay" will be available, not unlike the one you see when you play a PC game in Steam. When signed in under your Steam account, the overlay will track your progress, let you know your friends' online status, and immediately support Steam's cross-compatibility features, including overlap between Steam Achievements and PS3 Trophies, as well as "Steam Cloud" saved game syncing, letting you continue where you left off on either platform.
PS3 copies of Portal 2 will also include a code to download the PC/Mac version of the game through Steam, a feature that was previously known. Basically, if you're buying Portal 2 on PS3, you're covered. The game is out on April 19.