You might also want to consider some new solutions made by Parallel Universe (my company). We provide an embedded, in-memory spatial database called SpaceBase, that also helps you parallelize game code, and an open-source data distribution layer called Galaxy. These solution have been built specifically for MMOs (and some other, similar applications) and work at extremely low latencies. They are not as complete as some NoSQL databases, but that is precisely because we realize, as ApochPiQ said, that to handle specific game requirements there will be some problems you will need to solve yourself with ad-hoc solutions, but we try to help with the really hard general problems.