I think it's up to Pandora to actually make a plug-in that will work with Vista Media Center and be compatible with extenders. It's a great idea, however, bear in mind that international users won't be able to take advantage of the service, at least not the last time I tried to stream music with the them. I believe the service is for US listeners only.
However, in the meantime, you can install Radiotime (www.radiotime.com). It will give you a good choice of radio stations what will work with extenders.