Thanks again, everybody.
I suppose all that remains, now, is to wait for the new Mac Pro. As I understand it, the new server-class Sandy Bridge CPUs are expected February/March so - if there is going to be an update - it'll happen around then.
If they don't update the Pro, well, I suppose it'll have to be another iMac. I've toyed with the idea of going back to PCs, and retaining my MBP for OS X matters; but I really don't want to do it. Here's hoping Apple don't force me to revisit that question...