I clicked my screen options to show 1 column view in Post / Add New area. I then went to click back to a 2 column view and it does not go back to 2 column view. Checked with both Safari and Firefox as I’m using a MAC. Tried it on a Windows laptop as well. Same problem.
I tried re-installing WordPress and even Updating the Network. Still no change.
I just duplicated your symptom using WP 3.1.3 and Firefox 4. My solution was to reselect the 2 column layout, then drag and drop the Publish, Categories, and Post Tags boxes back to the right column, releasing them into the rectangular boxes/empty place-holders that appear.