Technically I don't see why not but from a monetary point of view surely it would be madness for Sony to provide that as a feature. Unless you mean you need a PS4 to stream to PS3 at which point I fail to see the, er, point.
I'm thinking more along the lines of using the PS3 to demo the PS4, really, which I think does make monetary sense - that way you don't alienate most of your existing users by "abandoning" them on PS3, and you show them what they're missing with the new hardware (albeit in compressed, laggy form). An example: you buy a PS4 game on PSN (perhaps after trying a demo first), stream it from Sony's distributed servers, and if you do end up getting a PS4, that game is still yours to play, only directly on the hardware now as it's tied to your PSN account (as all PSN purchases are).
Streaming is not quite good enough for serious action games, which is basically (almost) every game that "matters" in a monetary sense, so "hardcore" gamers won't be satisfied with the experience (initially perhaps, unless they slow things down, much like FPSs are already slowed down on consoles because of the control scheme), which provides an incentive to upgrade. Whatever the "hardcore" contingent is seen to be doing, the mainstream will seek to emulate (with some priming from the industry and attached media), after a delay, probably.
It just seems like the right thing to do, to me, even if it's only offering PS4 demos and not full games on PS3, through the PSN. I have no idea if the PSN streaming will be ready to go at launch, but the PS4 streaming is being touted loudly enough, and Gaikai already does / did what I'm describing.
Forza is probably considered a better game because they not only got Porsche (even if the consumer paid for it) but also acquired accurate tire data from Pirelli.
Where does the tire data come from in GT5? Somewhere as "cool" as Pirelli?
I was talking in broad terms. Of course tyre data isn't cool (although Porsche obviously is - and then again, pretending that tyre data matters to you might appear to be the cool thing to do), but I seem to remember the blurb about it tying into the same "we're car guys, we're cool" marketing that preceded the most recent game. Besides, these things only make the game "better" if they are the things that matter to you; "cool" things seem to matter an awful lot to an awful lot of people.
It's not a criticism, by the way, just an observation based on the way a lot of people (not many appearing to have any real clue, I'll admit) seem to justify why the one is better than the other. But this isn't the place for it, I guess, so I apologise.