Looks like the reports that the PS4 won't be backwards compatible with PS3 games is only half true. According to the WSJ, you won't be able to stick your old discs in, but the PS4 will be able to stream old PS3 games instead, like a PlayStation-only OnLive.
Sony bought one of OnLive's rivals, Gaikai, last year, which gives Sony the power to emulate the games on its servers and stream them to your PS4. Of course, there's going to be lag -- you just can't avoid it -- so I wouldn't fancy trying to play Call of Duty across the system. Then again, there's nothing stopping Sony pumping the same streamed-PS3 games to its tablets and phones too, so perhaps this will end up being a tour de force for Sony.
While I'd prefer to just be able to stick my library of PS3 games in the disc slot and go, if Sony does us a solid and allows us to submit the serial number from our existing game discs to get a streaming version, that'd be a lot better than nothing. I have a feeling that's not going to happen though, and you'll just have to buy them again from the PlayStation store. Or you could just keep your PS3, I guess. Radical thinking, I know, but it just might work. [WSJ]
Updated: Apologies, we accidentally brought you this news...twice.