What do you mean? Dropbox syncs files amongst the computers you own and that is the most used feature I think. It is a sync. (In fact before using Dropbox I was using rsync over ssh.)
Alice has a file. Bob has a copy. They both edit it, and want the edits to be synchronized (by some sort of smart merge). That's the over-engineered syncing solutions Joel is talking about. Not just rsync.
Sorry about the semantics ... I should have been clearer.
