The Steam integration sounds like a dream, but when you start to break it down, it might not be quite as useful as it initially sounds. For starters, the save games aren't compatible across platforms, so your cloud-based save for the PS3 version won't pipe into the PC/Mac release. Perhaps that was a bit much to expect in the first place. Of course, PlayStation Plus subscribers already have cloud-based save storage across all PlayStation 3 games. Also, since the PS3 version of the game will tie into your Steam account, you won't be able to use your free PC/Mac copy to play co-operative games with only one copy of the game.
Anyway, all of these Steam features will require a Steam account linked with a PSN account, so you’ll need both to take advantage of these networks to use the Steam overlay. It sounds like Valve is making account creation easy for you PS3 owners, too. You'll be able to get yourself a Steam account directly from Portal 2.