Basically, you click the little "info" link* (the at the beginning of the track in ooTunes), then click the "Play in iTunes" link which will download a .pls playlist which can be opened in iTunes to play that stream!
The latest version of ooTunes will let you listen to any of the internet radio stations compatible with ooTunes right through iTunes (even remotely) so you can pipe them to your stereo connected by Airtunes!
In fact, it will probably even let you listen to whatever stations you want (this includes last.fm streams, internet radio, etc.) on your appleTV as well (though I don't have one to test it on!). If you have one, test it out and let me know if it works or not!
* You may also see the little "$" link there in the top corner. That's so you can search for the lowest price on that song or album, if you were interested in buying it. One day I hope to have that working with the currently playing song on a station, but the makers of that technology have been pretty unresponsive to requests from a little guy to license it. Just FYI.