Warn when parents != tip?

[Keith Edmunds]

I've recently converted from svn to hg. Developing on two systems and
pushing changes between them, I've been burnt a couple of times because I
haven't done an 'hg up' after having received a push. I understand what it
is going on, and why, but has anyone come up with a warning when working
in a repository where the parents are not the tip? I'm imagining some
clever bash prompt or similar.

Thanks.