UPDATE: iScroll2 is a full-featured solution that doesn’t rely on the method below. I recommend it but will keep this outdated guide for historical purposes: http://iscroll2.sourceforge.net/

a software hack called “sidetrack” has been available for some time, but it is shareware. if you already have it, uninstall it before proceeding with this new free method (which is actually based on apple’s public source initiative).

• check to see if you’re eligible for the new driver by running the command…

  ioreg -l | grep "W Enhanced Trackpad"

  if the command returns a line containing “W Enhanced Trackpad”, the driver should work for you.

• download the binary compiled kernel extension at this site
• open Terminal.app (probably in the Utilities folder of your Applications folder)
• cd to the directory where you downloaded the kext. if i recall correctly, safari automatically unzipped it for me.
• correct the permissions on the extension by running…

  sudo chown -R root:wheel ./AppleADBMouse.kext

• unload the current mouse driver and load the new one by running…

  sudo kextunload /System/Library/Extensions/AppleADBMouse.kext && sudo kextload ./AppleADBMouse.kext

  if your machine doesnt give you a kernel panic, you should be able to try out the scrolling in safari or the like. if it works, it’s time to make it a permanent deal.

• cd to /System/Library/Extensions
• move the old mouse driver to a backup by running…

  sudo mv AppleADBMouse.kext AppleADBMouseOLD.kext

• cd back to your download folder
• move the new mouse driver into the place of the old one by running…

  sudo mv ./AppleADBMouse.kext /System/Library/Extensions/AppleADBMouse.kext

• reboot and see if the changes holdthat should be all there is to it!