This is a really cool idea.
Unfortunately, I believe that since most maps use completely different art styles, the way that Photosynth goes about matching parts of the images that are given to it won't really work for matching different maps together.
In other words, if you had two different maps of the same area and gave them both to Photosynth, Photosynth would probably identify all the corners in the roads as parts of the images to pay attention to, but when it examined a small square of imagery around each corner or blob, the area surrounding that feature wouldn't match to the texture from the different map. I'd be delighted to be proved wrong about that, though.
If you happen to know of existing maps, you could always use MapCruncher ( http://www.microsoft.com/maps/product/mapcruncher.aspx ) to add them to Bing Maps on your own, to then live in the same world as photosynths and panoramas as Read/Write World ( http://bit.ly/readwriteworld ) gears up.