Originally Posted by PENDRAG0ON
Thanks to the PSN, PS3 compatibility is pretty much required now. The only way to properly exclude it would be an add on. And really, eventually it would just be combined with a hardware revision anyway.
Some time in 2013 the PS3 is getting another die shrink (from the 40nm range to the 20nm range) so costs will go down even more before the PS4 launches.
Where BC is concerned, there are no guarantees. No company has a good record in this arena, and I don't think the existence of PSN guarantees anything. Just look at the abysmal track record for games that should by all rights work just fine between PSP, PS3, and PSVita. It isn't just about software compatibility. Licensing and business decisions (the ability to make more money on the same titles) play an even bigger role. And much of that is out of Sony's hands.
The transition to the next console will only exacerbate those backwards compatibility problems. I'm sure Sony will do what they can to make sure there's as much BC as possible, but it won't be 100%. And based on prior history, it could be far, far lower than that.