Game Center's GKTurnBasedMatch seems to provide a pretty robust service for handling turn-based games, and OpenFeint appears to have something similar. The problem is, I'd like something like this for the desktop. I was thinking of rolling my own REST-based service on Rails, but after looking at the GameKit documentation, I realized it would take longer than I'd like to make it solid. I don't suppose something like this already exists that I missed in my searches?
Not really, I think you would want to program all the networking stuff yourself, I would make turn based stuff like so:
Again, just an idea I thought you could use, hope it helps.