I'm currently working on the Media Center gadget for Windows SideShow.
We've just put up the beta at http://connect.microsoft.com/site/sitehome.aspx?SiteID=312 so be sure to create an account and download to try it today! Feedback is encouraged so please let us know about any issues or comments and be reading through them as we work towards the final version.
With the gadget you can control Media Center, including seeing the program guide, recording shows, viewing your photos, browsing through music, and more. I use it on a Vave 100 remote control but you can always use the SideShow simulator if you don't have a physical device. (Get the simulator by following the links at http://blogs.msdn.com/sideshow/archive/2008/05/29/announcing-the-windows-sideshow-managed-api-sdk-and-runtime-releases.aspx)
Update: Stuart at The Media Center Blog has a lot of screenshots and detailed information about the gadget. Check it out at http://windowsmediacenter.blogspot.com/2008/07/windows-media-center-sideshow-gadget.html
Update 7/7: The direct url to enroll in the beta is http://connect.microsoft.com/InvitationUse.aspx?ProgramID=2380&SiteID=312&InvitationID=GPB-79MM-RFJ2