As a Mac user, I have been very happy with the performance of the open-source VLC player with AACplus, and if you set things up right, the AACplus stream can be backward compatible with QuickTime (installed on all Macs, and included with every iTunes install on a Windows machine). In addition, there is of course an easily downloadable codec for Windows Media Player on Windows machines that allows WMP to play AACplus.
The backward compatibility lies in the fact that QuickTime will play AAC, but not the "plus" portion.
Look at SomaFM. They seem to have set things up so that their AACplus streams can be played in QuickTime, and then (optionally) I can open the stream in VLC to hear the "plus" - higher sound quality. However, using QT as default makes it much easier for Mac users since you can safely assume they have QT installed.
For example, with Groove Salad:
http://somafm.com/gs.mov will play the AACplus stream in QuickTime, but only the "AAC" portion is heard (Stereo, 22050 Hz).
By downloading that .mov file and opening it in a text editor, I find rtsp://64.202.98.91:554/gs.sdp - entering this URL in VLC allows me to hear the AACplus SBR full sound quality (Stereo, 44100 Hz).
In addition, the same URL (rtsp://64.202.98.91:554/gs.sdp) can actually be played in RealPlayer for those who prefer it. (I am not entirely clear whether the full AACplus sound quality can be heard with this method. I suspect it may depend on the particular version of RealPlayer used, and could even be different on Mac & PC. But hey, the stream is listenable.)
For another example of a great streaming setup, have a look at Z 88.3 in Orlando -
http://www.zradio.org/ .
Z 88.3 uses the same exact AAC stream, as described above, for both QuickTime and RealPlayer users, as far as I can tell, and it works well. They do also provide a (non encrypted) WM stream. As far as I know, Z does not use AACplus, just normal AAC. Not sure why, but I can't get their plain AAC stream to play in VLC. It works fine in QuickTime & RealPlayer, though.
In summary, I highly recommend that stations broadcast online in AAC. In my opinion, especially when AACplus is used, quality is much higher and artifacts are much fewer than with Windows Media. Even plain AAC, though, can be quite listenable with Mono at 32k or below, and Stereo at 64k and above.
I really don't know why they haven't yet done so, but I fully expect Apple to eventually transition the iTunes Radio section to include (or perhaps exclusively feature) AAC broadcasters, rather than plain old MP3. Apple of course promotes the most popular AAC music store (iTunes) and portable AAC player (iPod) and owns a chunk of the AAC patent portfolio.