I've been thinking for a while that (on the Mac, at least) simple single file revision control would fit nicely into the now-unused (but unlimited) file forks, using extended attributes for the tags.

Since I don't write any major apps, and this feature would be mostly ignored as a stand alone 'manager' app, I haven't tried implementing it. But if it were just a standard feature of all Document based apps, I bet it'd see a lot of use, and people would wonder how on earth they got by without it.