Mac OS X Hints uncovers a cool feature in Tiger that allows you to turn your landscape display into a portrait layout:

If you go into the Displays preference panel, you will see a new drop down called “Rotate.” This will rotate the image on your display a number of diffrent ways: 90, 180, or 270 degrees.

This looks particularly cool (if not that useful) on my PowerBook, and one can imagine it coming in quite handy with say, a tablet interface we might see as an option in a future version of OS X. But be careful using this trick with an older (VGA CRT) monitor connected. They report getting a “scan-out-of-range error” they were unable to resolve.