This is the reply I received:
Thanks very much for writing. Supporting more Windows Mobile devices is definitely something that's on our radar, but for several reasons, it's among the more tricky smartphone platforms to support. Unfortunately we can't do everything we'd like to do at once. It will probably be some time before we have a universal Windows Mobile client ready.
You can find the list of currently supported WinMo devices here:
http://www.pandora.com/on-windowsmobile
Thanks for your patience in the meantime.
thanks, Jonathan Pandora Listener Support