This is pretty trivial, but on my 1.67 GHz 15" PowerBook, I've just noticed that I can scroll by keeping one finger stationary on the trackpad and moving the other one. In other words, as long as the trackpad senses two fingers on the trackpad, it will scroll even though one of them isn't moving.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050505215227532