From my reading I found that there is actually technical reasons why the PS4 wont be backward compatible. The PS3 is a PowerPC based and the new PS4 will be x86. Supposedly in order to play PS3 games it would have to emulate a PS3, and I dont think the PS4 has enough horsepower to emulate the PS3 and run the rest of the PS4 OS...
This is right, emulation is a pretty compute-intensive thing and there's no way the PS4 will be powerful enough to emulate the PS3. They would have to put in dedicated hardware for a feature few would use or care about, which would increase the price of the console.
Last time I looked into it even the xbox and ps2 weren't properly emulated yet.