I just noticed blog posts from Michael Creasy and Ian Dixon that I want to make sure everyone notices. If you have an Xbox 360 that you are using as a Media Center extender, once you install the Xbox Spring 2007 Dashboard Update, you will be able to configure your Xbox 360 to boot directly to your Media Center.
To access this new setting, go to the Xbox Dashboard, then the System tab. Under Console Settings, select Startup, and you will now see 3 options: Disc, Xbox Dashboard and Media Center.
If you use your Xbox 360 primarily as a Media Center extender, this might be a really useful option for you to try out.