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

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.