It's not like the PS3 had PS2 backwards compatibility for very long - hardware support was phased out within a few short months, and software emulated support within a year[1]. I'd say it worked out pretty well for them thus far.

[1] http://en.wikipedia.org/wiki/Ps3

They hinted at "Gaming in the cloud" that would support PSX, PS2, PS3, PSP... etc

Likely using the same tech they use for playing a PS3/4 on a PSP/PSVita (respectively)

Real backcompat with cell would be really, really hard. It would either cost a lot because you have to put an useless cell cpu in the box, or cost a lot because you have to expend the engineering resources to redevelop the games you want on a new cpu.

However, the platform is sufficiently close to the PC that shipping the PC versions with slightly tweaked GPU code should be relatively painless. PS4 should have a huge library at release.