I've just rediscovered the iTunes radio station section after a couple of years, and I've stumbled across a few good stations I've never listened to before. I've had Morow (the best progressive rock of yesterday and today apparently) on all afternoon and they play some good stuff.
They've got a website at
http://www.morow.com/ for those of you who haven't listened to this one before. As with Planet Rock you can listen via the website or iTunes if you have it. I did find another station but their sound quality is rubbish (slow streaming rate) so I'm not going to bother plugging them.