I have turned off all plugins and gone back to the default theme. But somewhere along the line - I've not made changes to the sidebars in about a month - I lost the right side of the Widgets Administration Panel:
<img src="http://dragonflyeye.net/files/2012/10/Screen-Shot-2012-10-26-at-1.15.16-PM.png" />
No idea what caused this or how to fix it, especially given that I've eliminated the more obvious culprits? Even with my normal plugins running and my preferred template used, the front end works fine. I'm only prevented from doing this one thing. Weird!!