Here is a plugin request related to this idea. I'm crossposting it here on the forums to find out whether anyone would be interested in working on a plugin for it.
Two things are needed:
1. the ability to flag a certain revision as the stable version
2. an interface to easily revert to the stable version
Basically, stable versions are privileged revisions. This would be an important improvement to the current flat system where it is not even possible to see at a glance which revision you'd want (because of the lack of edit summaries).
This would be a feature that would make WordPress way more useful as a CMS. I'm thinking of clients who want to fool around on pages, but with the assurance that they can safely revert to a stable version if they mess up.