I just noticed this and I think its new to iTunes3. If you simply right click (or control-click) on the column headers (where you would drag them around) you will instantly be able to edit which columns iTunes shows and doesn't show. Very handy.
Also, if you right click on any MP3 file you get some extra handy commands like "Reset play count" and others that I can't find elsewhere.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020724084257818