Open up a terminal and type:
UPDATE (8/13/11): The package isn't installed by default in 11.10 Oneiric Ocelot, so you'll have to install it first, either through Synaptic package manager or by typing
sudo apt-get install gpointing-device-settings
Select the TrackPoint device from the pane on the left. Mine is called "TPPS/2 IBM TrackPoint." In the middle of the window, you'll see a checkbox labeled "Use wheel emulation." Check the box, along with the ones labeled "Enable vertical scroll" and "Enable horizontal scroll" as you see fit.
You'll also need to select your scroll button from the pull-down menu. In my case, it was button 2, but YMMV.
While you're here, you can also switch to the TrackPad options and disable your trackpad, if that's what you're into.