Nice idea. Sadly we haven't yet worked out how to tell light to pretend it's hitting something solid when it isn't.
I like the other idea further down the thread. It could start out as just a way to review trialsy locations with other users of the app. It could then serve appropriately tagged photos/videos from twitpic, flickr etc for a given location. It could be a relatively straightforward HTML5, Google Maps API and database affair, usable on any decent smartphone.
Hell, I might even make an app like this when I get some time.