Microsoft notifies via email of the release of  “Media Center Gadgets for Windows SideShow Release Candidate!” They’ve added support for Media Center Extenders:

Extenders are now supported if you are using a SideShow device that can pre-enable the gadgets. The Ricavision Vave100 remote control is an example of a device that pre-enables the Media Center gadgets. Try this with your Xbox 360 or other extender device and provide us with any feedback.

More info: Download | Installation Instructions | Gadget general information and known issues