I know this works in iPhoto 5 and iPhoto 6, and I haven't seen this documented anywhere.
You are editing a photo, and want to crop it to a fixed ratio using the Constrain pop-up menu. Your photo was taken vertically, but you now want to crop it horizontally. iPhoto doesn't realze this, and it is difficult to get the crop you want ... until now.
Hold down Option while selecting the area, and the orientation of the selected area changes from vertical to horizontal. This works going from horizontal to vertical, too.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060510173900135