That whole "back up a part of history so that you could undo stuff later" will be gone as soon as Changeset Evolution [1] is ready. An when it's ready, it will be tremendous.
Ooo, being able to push rewritten history, that's nice! Seeing that neither Git nor HG handles pushing history rewriting gracefully right now, this would be awesome.
[1]: http://mercurial.selenic.com/wiki/ChangesetEvolution