Actually, i don't see the point of BC with PS3. It only makes console too complicated and stop progress because you cannot change architecture much without losing BC. You can continue to use PS3 for PS3 games at the same time as PS4 for newer games.
It's simply impossible to emulate Cell CPU on x86 CPU in real time. Putting Cell CPU as second to x86 is lame too because to much heat and high price. At the end, PS3 didn't die without PS2 BC. Most likely, very popular PS3 games will be re-released for PS4.