XM Online uses Windows Media for it's playback, so it will only work on a PC/Mac or Pocket PC/WindowsMobile. I do have a solution that works and lets me listen to XM radio on my 770, on my Treo 650, and pretty much any Shoutcast-compatible device.

I use a program called Nicecast on my Mac. It allows you to set up your own Shoutcast server or feed, and then it gives you an IP address from which you can access it. I load up XM Online in Safari (web browser), and then have Nicecast stream that audio as a Shoutcast stream. Once that is done, i can tune in from the 770, and I can even use the great VNC Viewer on the 770 to change the station.

Sometimes I forget to even start the streaming before I leave home, so VNC allows me to get in, start everything up, and then start listening to XM online...

What's cool about Nicecast, is I can stream from any app, whether it is iTunes, RealPlayer, Quicktime or others. And the VNC Viewer enables me to go in and change sources whenever I want.