After upgrading to 2.5, I went to write a new post, and I also wanted to add a new category while doing so. Except I couldn't. I could get as far as typing in the new category name, and clicking the 'Add' button, but it didn't do anything. Did a fresh install in a different path, just to test, and it worked there. So I disabled all my plugins, and then it worked again. I finally tracked it down to wp-openid, which makes me sad, as it's one of my favorites (although not used very much on my site). I deactivated, and re-activated the plugin several times just to make sure it wasn't a fluke, and it was consistent every time.
Let me know what (if) I can do to help track this down.