"Lion even notifies you of version conflicts and allows you to resolve them when you open the document."

Versions (the feature that was added to Lion) would be a good fit. (It however doesn’t seem to be used in that way currently.) There is no one correct way of handling conflicts, how to best resolve them depends entirely on context, context only the user is aware of. That’s why this problem is so damn annoying.

I nevertheless believe that it’s possible to find some heuristics that do the right thing most of the time – without ever having to annoy the user with dialogs. Easy to use versioning (where all conflicting versions are still available as versions) covers all other times.

git never did...