If it's anything like the PSP and the PS3, you'll be able to create up to a hundred save files (I think that's the limit?) for all games.
I've had no problems sharing my PSP with my brother, despite never bothering to create different accounts or anything like that.
Of course, I'm still in the "this sucks, stop being so backwards" camp. :p
Edit: I have no idea how this would affect multiplayer, though.