Well, I guess it wasn't that simple then.
I have a test login for which karma now works. The only difference between that user and you all is that your accounts were created many moons ago, before the Great Forum Upgrade.
So my theory is that -- similar to the song rating thingy -- these two facts are related. Can anyone with a new account (and more than ten posts) confirm that karma is working for them? It should look something like this if it is working:
[ ] Offline
Clicking on the [yay!] will give someone +1 karma. Give one to DanO or m@w and see if it works please... Thanks.