I attached my Dell 1703FP to my new MacBook (1.83GHz) via mini-DVI to DVI, and noticed a new option in the Displays System Preferences panel: Rotate. I tested it, and it works as expected.
Those integrated graphics that everyone is slamming seem to do the trick! I hadn't heard anything of this previously, so apologies if this is old news.
[robg adds: There's no mention of this on Apple's MacBook pages.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060606072037300