I noticed this on the new 2005 PowerBooks ... if you are using FireFox (maybe other browsers), try siding your fingers left to right versus up and down to get a "page back" and "page forward" command. Gotta love Apple sometimes.
[robg adds: I can't verify this one...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050325102455672