Difficult to say because I changed many passwords anyway - so if the systems were reasonably sophisticated, they'd clock that and not email me.
I've noticed, though, that Paypal (who were apparently not themselves affected) have just hammered out a change to the user terms and conditions covering (among other things) responsibility for data used by third parties - so I would guess that there's a lot of scrambling around behind the scenes in many firms.
Difficult one for them to call, reputationally - admit you were vulnerable and have fixed it or try to hold the line but run around like disturbed ants behind it? I find it hard to believe that only MN and some Canadian tax authority (?) were the only ones potentially affected.
All in all, I think Mumsnet took the right approach.