I had the exact same problem.
Do you run Thesis? I am. Don’t know if that had anything to do with it. I also unplugged all my plugins. I was tearing my hair out.
Here’s how I got it solved. I had upgraded (I thought!) to WordPress 2.9.1 automatically using the dashboard… but when I logged into GoDaddy and got to the Hosting Connection screen, to my surprise, it still said I was running 2.8 and did I want to upgrade to 2.9.1.
Bingo. My dashboard said I was running 2.9.1 but my host said I was still on 2.8. After I upgraded from the GoDaddy Hosting Connection interface (it takes a few minutes to actually go thru) it fixed itself.
Maybe that will help.
Chris