Microsoft Research has been working with Virtual Earth to develop a cool new application for overlaying information in the Map Control. Chris explains it better than I can:

"The basic idea is that you can take any map in PDF format and overlay onto a Virtual Earth map. The benefit being that you can have your own specific map with the cool Virtual Earth draggable functionality, double click to zoom in, pan, etc. As an example, I can take a PDF map of Disneyland and overlay it onto a Virtual Earth map. So, while I'm flying around Virtual Earth I can also explore the Disneyland park map."

For instance, this same technology could be used to overlay information specific to your organization on the map control, such as the general routes of infrastructure like subway tunnels or gas pipelines in an emergency situation.  You could also use it to bring ski trails and bike trails into a real-world environment on a an adventure site.

You can find out more here: http://research.microsoft.com/mapcruncher/

And, see the press release for more info.  

PS - Because it requires a slightly modified Map Control to work, the application is released in a research prototype form only and no support is provided. Use it in a production environment at your own risk.  But, isn't it cool?