We will continue to support the Hosted HTML development model even though new things are coming down the pipe for Media Center in Windows Vista. We posted a couple of resources for those who will continue to push the envelope for HTML applications in Media Center.
Developing Services and Applications for Microsoft Windows® XP Media Center Edition 2005 is designed to provide an in depth, yet high level overview of developing an experience for Media Center in Hosted HTML.
The Powerpoint Presentation Overview of the Windows XP Media Center Edition 2005 SDK is a companion piece with a bunch more visuals. It also delves briefly into Media Center Add Ins as well as the Click To Record features. It's perfect to give your design and development groups an introduction to creating experiences for Media Center.