Grooveshark has had[0] a very sharp HTML5 interface for several months now. Here's a story[1] on HN from 220 days ago with discussion about it. Pandora is very late to the game.
Grooveshark is already available with HTML. They use flash behind the scenes to play back the audio, but that's it. Pandora will most likely be very similar.
Didn't know that, thanks. When I go to grooveshark.com I get "We had a problem loading Flash." on a mostly blank screen, which is pretty much exactly what I get on RDIO.