If you used Migration Assistant, or upgraded from a previous OS X release, the Appearance System Preferences panel settings are retained. As a result, you may not have noticed that the minimum for anti-aliasing is now 4px, down from 8px in 10.4. Fresh installs of Leopard have 4px set as default.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071206033227688