I came upon this hack today and have applied it with complete success over power level 9000. Basically, the new MBP and the MBA both have gestures using four fingers that are possible considering the hardware inside but disabled by Evil Apple on the previous-generation of MBP (namely, my Bellamy’s generation). With this hack + OS X 10.5.6 and higher, this functionality is possible. The hack is painless and dead simple.
If this at all sounds like anything you’d be interested in doing to your previous-gen MBP, click here for the tutorial and file download.