I cheated. The "real" way to take a shot like that is to use a slow shutter speed and track the object you are shooting with the camera. Some Canon lenses (including the one I used) even have a panning mode where they stabilize the image except in the direction that you are panning. Sadly, I've never been able track well enough to pull it off.
My cheating solution was to take a relatively high shutter speed shot. That left me with an OK shot of a car frozen in mid air between the ramps. I then copied the car to another layer and added a motion blur to the background. To give it a bit more realism, I also cut out each wheel, applied a radial blur to it, and pasted it back on.
0bli0 has some great panning shots in the "Show me: photos an avg tourist may not take" thread.