Two finger scrolling didn’t exist until recently but there’s a helper for pre-2005 Mac portables. iScroll2, brings two finger scrolling without the need for any hardware modifications. You’re up and scrolling after a short download and installation. Most models of Mac portables are supported.
“Models that are known not to be supported include all G3 and Titanium PowerBooks, the very first 12″ PowerBook model, all G3 iBooks, and all newer machines that come with two-finger-scrolling built-in: The trackpads of the older machines simply lack two-finger detection capabilities completely, whereas the models that have scrolling built-in use a completely different, USB-based trackpad, the driver for which is not available as open source.”
iScroll2 is donationware.