You may have heard there was a July'07 refresh of silverlight to the original Mix'07 release.
Unfortunately, this refresh breaks the Visual Studio Silverlight templates from Mix, and my first Silverlight example falls victim. Joe Stegman has a great list of the breaks. There's also a list of the breaks in the Silverlight SDK in "Changes from Alpha.docx" (thanks to Jon Langdon for pointing this out to me). And these are also discussed at the silverlight forums.
When you launch (F5) the project, the browser comes up but you get a "silverlight upgrade" image instead of your app.
First you need to update your tools stack. That wasn't too bad. Then you can make the adjustments to keep your Mix'07 projects running on the July'07 refresh.
The updates I needed to make were:
I've updated my sample at the original URL here. Move intensive projects may need to make larger changes to cope with the other breaking changes.