The iTunes radio stations can be dragged into the Library and playlists just like MP3 files. Once you've done this, you can edit the tags to organize them however you like. I set the Artist to "Radio Station," and the genre to whatever applies. Then I have a smart playlist of all songs which contain the artist "Radio Station." Turn browsing on, and you're no longer limited to Apple's categories. You can also download .pls files from a shoutcast server and drag it to iTunes to add stations that aren't in Apple's list.
I would recommend though, that you uncheck radio stations that are in your library. I haven't tried it, but I suspect that if you were playing your library on random and iTunes landed on a station that it would be stuck on it till you hit stop.
Because you can have radio stations in playlists you can use
this hint to wake up to your favorite station via iCal.