PS3
will not be able to play XBOX games:
Reason 1. They'd be sued
or
Reason 2. They'd be bought out
Plus, it is bullcrap that they'll make a "master system."
Too many companies would be suing their asses
The only different system that was made was Panasonic (I think). They made a custom GameCube that plays the GameCube games and also plays DVDs; I'm pretty sure Nintendo gave them permission to use the abilities to read GameCube games or they would have gotten pissed off really fast.
Anyway, I hope next-gen systems will all be backwards-compatible, since it gears more people to buy them, knowing their older games can be played on them (another reason to add on why PCs will always be better than consoles, but that consoles are still fun

)