This is going to make some of you nuts (it did me). None of these solutions worked for me. I spent all day on this and it turns out it happened because of a bonehead mistake.
Turns out that if you forget to deactiveate all plugins before an upgrade, this problem is the result. downgraded my wp installation to 2.9. Deactivated all plugins then upgraded back to 3.0.1. Now all is working fine!
Let me know if this works for you. It took me 8 hours to figure it out. Thanks to the awesome guys at bluehost who really went above and beyond in helping me!
