Yep - chimney supporting the joists is the problem (although I heard
that frost heave caused the chimney to lift - but same effect) and
de-coupling the joists from the chimney and adding any necessary double
joists and/or headers is the solution.

Interesting. So the basic problem is not that part of the house is
falling down, but that the other part of the house is being forced to
stay *up*? And the solution is to basically just let the part that's
being held up to settle down? (I was thinking I'd have to do the
opposite - to try to prop up the rest of the house around the chimney
to prevent it sinking further.)

This doesn't sound like it's really a major fix - am I wrong? What
happens when the joists are decoupled from the chimney and that part of
the house suddenly drops 1-2 inches? Or is this done as a slow process
with jacks (similar to straightening a sagging main beam, but in
reverse)?

And if this is really the problem (and it makes sense to me, based on
what I'm seeing), is it really going to matter if I just leave it alone
for a while? It's to the point where I'm sure the previous owners must
have noticed it and contemplated doing something about it at some
point, but they obviously didn't. Is it purely a look-and-feel issue
then?