Like many of you, I use the Tidyplates addon when tanking as it makes it very easy to see who you have aggro on and who you need to taunt back. It's excellent. Since the 4.01 patch however it's been broken because of the issue of overlapping nameplates - rather than staying next to the mobs they represent, the nameplates wander around the screen when tanking large packs, which makes the addon pretty much unusable.
The problem appears to be due to changes Blizzard made to the default UI, and I've discovered that entering the command
/console spreadnameplates 0
should fix the problem. I'll check this later when I get home.
If you don't use Tidyplates, here is Kadomi's review some time back.