Interesting stuff. I'm working on a system that we think will use version vectors. There are some other ideas like Interval Tree Clocks out there, but they look more difficult to translate into something that can reside in an SQL database. For that matter, it's been a tough challenge trying to translate some of these ideas into something that can work in practice, over HTTP, be sort of transactional in its behavior, and be able to work with single records at a time, rather than trying to transfer everything at once.