I recently convinced my sister to buy my beloved MacBook Core Duo, while I upgraded to the MacBook Core 2 Duo. Being lazy, I just swapped the hard drive from my old CD to the new C2D. I ran into two problems on my C2D:

Seaching the net revealed no clues. Except a linux kernel patch. It turns out the CD uses the Geyser III trackpad, while C2D uses Geyser IV. I dived into this, for me, rather unknown Mach/OS X-territory (altho I consider myself a long time unix zealot), and located the file /System/Library/Extensions/AppleUSBTopCase.kext/
Contents/PlugIns/AppleUSBTrackpad.kext/Contents/Info.plist. My old hard drive, from CD, contained no information of Geyser IV. Stupidly trying to edit this manually, and reloading the kernel extention (using kextunload and kextload) was, of course, unsuccessfull. But, behold!, copying the AppleUSBTopCase.kext-directory from the new hard drive fixed everything. I am now happily scrolling using a new MacBook C2D using the original OS X installation from my old MacBook CD.