Now, if you really want to see the route of that bike trail you can trace it exactly over the satellite image. Or if you take a national park map you can see the exact route of the hiking trail into the mountains. It is simply phenonenal given the simplicity of its use.
The site has several examples, but my favorites were how the developers, who are avid fliers, took their own digital photos out of an airplane window and then published aerial versions then those that already existed for a city called Forks in Washington state. The other MS internal one superimposed floor plans for MS Campus buildings onto Virtual Earth. We were left to consider how this technology can easily enable companies to not only show you the store that your item is in, but exacly the shelf and location in the store. A dream if you have ever visited a Fry's superstore to try an hunt down some elusive piece of computer equipment.
I expect to see a large number of mashup sites now given the extreme simplicity of this tool. Now I just need to dig out some electronic maps and get to work!