I've had one half built sitting on my hard drive for about a year. Perhaps I will take it in a less creepy and more anonymous direction than what the color people are doing.
It has a horrible (or awesome!!) name and uses GAE as a backend. Which between the blob store API and the geo libs has everything you need to build and host this type of app really cheaply.
So for anyone trying to do this over the weekend I would start with that. The tipfy framework will get you moving on GAE really quickly as well. (tipfy.org)
