There is an interesting article on the BBC today about some researchers using the Xbox GPU for parallel processing.
This would actually be fairly easy to do with XNA, though I would be really interested in seeing the types of HLSL they write for the algorithms. The only difficulty, as I see it, is that XNA doesn’t have a generalized API for data transfer to a server. The networking API’s allow you to communicate between other clients in multi-player games but they do not let you upload results to a server. They might let you upload top scores and a few other minimal stats but certainly not the large datasets these researchers are looking for. It would be really cool if the XNA team created a complimentary server client that would allow 3rd party indie game developers to create server software to enable this type of application as well as ones that might involve persistent worlds and things like that.
I would be very interested in something like that.