PlayStation Now currently only offers PS3 games.
But that could change in the near future. According to what Sony Entertainment Network vice-president Eric Lempel told Game Informer , Sony intends to expand beyond the PS3 platform:
"In our plans going forward we’re looking at everything so there’s the real possibility that you’ll see PlayStation 1, PlayStation 2, and PlayStation 4 titles available. Right now it’s just PlayStation 3, but these are all options for the future."
Lempel added that Sony is examining possible changes concerning the price model. User feedback hasn't been entirely positive on this score; don't forget that rental prices can run as high as $49.99, which is insane . You also can't access digital titles you previously purchased on the PSN, which is another problem. However, there are some rentals that go as low as $1.99 and hopefully, we'll see more of those in the future.
It'd be very cool to have games from all the PlayStation generations available, too.