I think there is a startup in this. A simple API on top of S3 accepting and pushing updates. Useful feature, easy integration, pain to implement well on hour own.
I think benjaminwootton was talking about solving the sync/merge problem..? There are still plenty of opportunities in that space, no?
It's easy to say 'your app needs sync', and I agree it _should_ be part of every app, but depending on what data you're dealing with, this isn't easy to do.