I noticed this when I was adjusting the screen height of my display (an eMac), under Geometry in the Displays preference panel. You can click on the image of the monitor and drag your mouse to dynamically make adjustments. (hover your pointer over it and notice the green box). May not be new, but I'd never noticed this before. I usually just used the adjustment buttons on the right.
[robg adds: I don't have a CRT, so I can't test this one.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030304214653179