While it's currently accessible on PS3 only, that changes tomorrow once the beta rolls out on PS4. Sony says more invites will be sent out to PS3 players in the "near future."
Streaming PS3 games to a PS Vita sounds much more interesting, personally, and I'm less concerned about the technology powering PS Now than I am its supported library and pricing. We'll have to find out about that stuff before too long, as the service launches this year.
PlayStation Now: PS4 Private Beta Starts Tomorrow [PlayStation Blog]