I'd wanted to hack google maps and replace the tiles with ones from Terraserver. The idea would be to be able to switch back and forth from street map to satellite images. It seems like this should be doable. The 2 main problems are determining the pixels per degree of the terra server zoom levels and dealing with UTM zone boundaries (using terraserver tiles you cannot seamlessly display a map that crosses UTM boundaries)
Instead I wrote a fairly simple bookmarklet that opens a window and displays the satellite image corresponding to the current map. When you zoom in/out or reposition the map, a new satellite image is fetched. This uses the Terraserver WMS which doesn't suffer from the cross UTM zone boundaries problem (but wouldn't be useful itself as tiles due to the slow fetch times).
Anyway, you can get the bookmarklet from my Google Maps wiki page. And here's a little screen capture showing the mall in DC:
