I was looking into this stuff a while ago - Darcs was actually my first distributed version-control system. When I started playing around with Git, I was initially a little dismayed that it didn't seem to have the history-changing abilities darcs has (apparently called "preparation branches" - I never knew that).
I asked around a little, though, and it turns out that "git rebase -i" essentially encompasses all those capabilities. The UI isn't as nice as darcs' until you get used to it - darcs has very well-designed, intuitive UI - but all the functionality is there.
I had heard about, but forgotten, spontaneous branches. They do seem to be very much like what Cat Dancer was asking for.
