MacCentral has a post that mentions that XM Radio adds Mac OS X support to their XM Radio Online. It looks like it will support any platform that can run Windows Media Player 9.0 or newer. This is pretty cool, as I listen to XM Radio driving to and from work, and now I can continue to listen when I am not in my car.
I am trying the free trial, and it sounds pretty good. It looks like the channels a are about 35Kbs streams for the low quality, and 67Kbs for the high quality. I am not sure how this compares to music that has been MP3 encoded.