I think SystemUIserver is implicated in this, but I can't be sure. At any rate, Force Quit in Activity Monitor does nothing. And that's when Activity Monitor responds at all. Sending a kill -9 to the process in question from the terminal window also does nothing.
I'm reduced to power-cycling the machine, in the best old-fashioned Windows 95 way. This is bad enough, but sometimes this causes the icons in the title bar to get trashed, and I have to go through and reset them all.
Needless to say, I am not particularly impressed at the stability of this system.