Well, I think it's a given that PS4 will have at least PS1 BC. I would imagine it would have to be because of the Classic collection. Making it PS2 BC would be wise, also, since it would save them time and money not having to build stand alone emulators for each PS2 game.
I think it was obvious it wouldn't play PS3/PSN games, though, as that would have added extra costs, probably pushing the console to $499-$599. Sony doesn't want to launch the PS4 at the same price as the PS3, as it would have to wait a year or 2 (after it could drop down to $299-$399) before sales would really take off. Of course, I imagine Minis will still work, as those are running in an emulated environment, already. Simpler game's code can probably be slightly retooled to work on the PS4. However, most games are probably just going to be run through Gaikai, so not really a big deal.
