It's really hard to imagine it will be able to play 360 games — not because of some evil scheme by Microsoft, but just because it's impractical. In order to backwards-compatible like that, you need one of two things:
A) Hardware that is extremely similar to the old system, so that you can either run its software natively on the metal or virtualize it without much of a penalty
-- This is how the Wii ran GameCube games
-- In a strange way, this is also how the original PS3 ran PS2 games — it included special hardware just for PS2 compatibility
B) Hardware that is radically more powerful than the old system, so that even though you have to emulate the old hardware, you're doing it fast enough to run the software at full speed
-- This is how the PS2 and PS3 play PS1 games
-- This is also how modern PCs do emulation of old consoles
Neither of these is the case for the 720 as far as I know. The hardware is very different from the 360, and although modern hardware is significantly more powerful than the 360, there still aren't any emulators for it or the PS3.
A) Hardware that is extremely similar to the old system, so that you can either run its software natively on the metal or virtualize it without much of a penalty
-- This is how the Wii ran GameCube games
-- In a strange way, this is also how the original PS3 ran PS2 games — it included special hardware just for PS2 compatibility
B) Hardware that is radically more powerful than the old system, so that even though you have to emulate the old hardware, you're doing it fast enough to run the software at full speed
-- This is how the PS2 and PS3 play PS1 games
-- This is also how modern PCs do emulation of old consoles
Neither of these is the case for the 720 as far as I know. The hardware is very different from the 360, and although modern hardware is significantly more powerful than the 360, there still aren't any emulators for it or the PS3.