If you seen Showtime on the Vista version of Windows Media Center you know how impressive Windows Media Center Presentation layer applications can be. They are light years away from the old HTML applications in terms of richness and are as Charlie Owen puts it "Full fidelity Media Center applications"
One of the new style of applications that launched with Vista (well in the US only but that is another issue!) was Showtime, offering access to access to premium content via the web all from within Media Center. Microsoft have published a case study on how the application was built using MCML (Media Center Markup Laungage) and C#
If only we could see more MCML apps coming out
Oh and watch out for interesting news regarding Media Center development from me soon
The Digital Lifestyle 
I agree Ian, they look great. Might want to remind your readers of the review I did a while back for the Showtime plugin for Vista MCE over at MissingRemote :-) if they want more screenshots/information
will do on next weeks show