I may not be the photography, but I can answer your question. This is accomplished by slowing down the shutter speed a bit and taking the shot while panning the camera to follow the rider. By following the rider, the camera basically sees the background/foreground moving and causes it to blur. It's pretty tricky, I've yet to get it right, let alone make it look good. This is an awesome shot. Hope that helps!

 I find it helps if you hold the camera, lock your entire upper body position, start the pan, press the shutter, and follow right on through until after the shutter closes again.

 Or you can have a bit of integrity and learn to do it correctly, like Duncan.

 I'd get a tripod out too!

 I've had bad luck panning properly with a tripod.

 Its 90% chance on a monopod; at 300mm (very zoomed in) handheld is unlikely. I agree with erik74; when panning at the same speed as the racer, he will stay sharp. Also, the lens used has a special kind of optical stabilization that only stabilizes the vertical shake to make panning images like this possible.

 No monopods here, usually pretty on it with the pans at the end of the season! oh and type 2 IS helps with the 300

 impressive Duncanp, especially to have such little vertical movement (as in none) without a monopod/tripod. hats off.

 @duncanp: please pm me back for pricing on print of this.

 Was just thinking the same thing.. Luckily I read the first few comments.
+1