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.
There are startups for that, i.e. Parse which he mentions in the post. And of course iCloud.
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.