I've had a couple of questions by email that I should answer (note I've paraphrased some of these):
What fonts should I use for my HTML Media Center application and can I use Flash? The SDK is where to go for design guidelines for HTML applications (the short answer is to use a sans serif font). Yes, Flash can be used in HTML Media Center applications - Napster and MSN Music both use Flash.
How can I add an application to the Start Menu? You can't. Well OK, you can, but we recommend that only OEMs use this functionality as there is a limit to how many applications can appear on the start menu and adding a new app will stomp on existing applications. If you intend to distribute your application you shouldn't do this, but if you really want to know, the details are in the SDK here.
I've seen the Media Center software for sale online is it OK to buy it? I'm no lawyer so I'm not going to comment on this. Media Center is an OEM product and only available with new PCs is the official answer though.