I have the same thing happening. I was on WP 2.0.3 and the only thing I did after making my last sucessful post was a small sidebar update, nothing unusual or that I haven’t done before. Then today I go to post and I get exactly what Jon described above. Figured I should upgrade to WP 2.5, which I have now done sucessfully…..and I still can’t post. My situation sounds exactly like Jon’s.
I’ve tried calling up the default template, which works fine, but the posting problem persists.
Any help would be greatly appreciated!