Niveus Media, makers of high-end Media Center PCs has opened up its Media Center Pocket PC application for 3rd party purchase without a Media Center PC.  With a WiFi-enabled Pocket PC, you can turn your Media Center into a dedicated Music Server and leave the display off during a party, or control a wide range of settings.

If you have multiple Media Centers around the home, you can control Media Centers there as well. A 10-day trial version is available here as well as additional information on the product.