PS4 Not Backwards Compatible
Shuhei Yoshida has given some very interesting insight into the upcoming PS4's capabilities regarding playing previously played games, but the clarification is a bit sobering after further confirmation from him.
While the PS4 will play previously-played games and used games, it will not be backwards compatible with prior Sony console releases. This means that it will not play PS2 game discs or PS3 discs, but also means that the upcoming console will not play games players have downloaded on PS3 over the Playstation Network.
There is a silver lining to this. The PS4 will be capable of playing these games via emulation. Playstation 3, for example, cannot play PS2 game discs (anymore) but can play PS2 games that are downloaded over the Playstation Store. A similar setup for PS4 should be simple enough, and seems to be well within the stated direction for the console.
PS4 could also have the games of these 'legacy' consoles available through cloud gaming. With all these possibilities, it's less painful knowing we won't be able to directly plug our older discs into Sony's new machine.