Did not know that. Well since Sony bought Gaikai I don't know their plans, but they could at least let you play the games you own without charging for them again. I don't know how complex this would be for Sony to manage, but it would be nice.The hardware is completely different. If Sony wanted backwards compatibility in the PS4, they'd basically have to throw a PS3 inside the console, which would be very expensive.
They tried that with the PS3 at launch, and it was one of the factors that led to the absurdly high price point ("599 US dollars!" etc., etc.).
Anyway I also believe the future of gaming will be 100% discless and on the cloud so like it is on Apples App Store you have games and apps to that specific account and can download them at will.