The PS3 is not supposed to come out until 2006. Sony wants to concentrate on the new PSP next year to help it across different markets.
They might have strong competition, Xbox has toyed with the idea of having its next system out by next Christmas, to get a 1 year head start on the next generation, much like the PS2 had on this gen.
PS3 has not been confirmed to be backwards compatible, but more than likely wont be. Sony will more than likely chose to use UMD to help fight people from copying thier games. Sony innovates on the digital media front, and I wouldnt even be surprised if they came up with even another storage device for PS3.
Clarifying and summing up, nothing is in stone on Backwards Compatability.