The PlayStation 4 won't offer backwards compatibility, even though Sony said it was a "tough decision" to ditch the popular feature.
You'll be able to utilize the new Gaikai cloud service to gain access to old PlayStation games, but the machine itself won't be able to play discs from past consoles. That means there's going to be a lot of games in your library you won't be able to play. Many find this to be, at the very least, discouraging.
So when you get the PS4, do you plan to keep your PS3? Or maybe the more accurate question is, do you intend to keep your PS3 hooked up? Me, I only see the PS4 as offering more of the same gameplay experiences, only with better overall performance, so the PS4 will replace the PS3 in my entertainment center. I've explained in the past why I can't see going back to play PS3 games, and why I'm much more likely to return to PS1 and PS2 titles. That's because there are gameplay styles and mechanics that have died out (such as turn-based RPGs) that I can't really find on the newer consoles. This is why my PS2 (which can play PS1 games, remember?) will remain hooked up right next to my PS4.
I just can't see much reason to keep the PS3 running. Why would I play Uncharted 3 when Uncharted 4 is out on the PS4? Why would I play Gran Turismo 5 when I could be playing Gran Turismo 6 ? The only thing that might dissuade me is future must-have PS3 exclusives, provided Sony has some in store after the PS4 releases. Don't forget that last generation, God of War II released on the PS2 several months after the PS3 was on store shelves. So what about you? What's your plan for your PS3?