Call me picky, but I'm pretty sure the motion blurred background is Photoshopped. I'd bet that the original of this image is either not moving or moving much slower than it appears.

The areas on the edges of the non-blurred foreground (kids, truck) look like they've been masked out to allow a blur in only the background.

Check out the mask on the right kid's knee in the larger version of the image linked by Esther -- see how soft the edge is there? - doesn't look natural. Also, look at the mud flap on the truck; the white paint stripe on the road blurs all the way through the flap. That wouldn't happen in-camera if the subject and camera were moving at roughly the same speed.

Additionally, the blurring on the trees in the background looks very much like Photoshop's motion blur rather than an in-camera motion blur. I say this because, in real in-camera motion blurs, the brighter areas of the blurred image will greatly overpower the darker ones. (think of long exposure night shots of cars driving -- the headlights always streak) However, here, the darker brown areas seem to have just as much emphasis as the bright areas, which is what you get when you add a blur in Photoshop.

The turning truck wheel could be done with a radial blur.

Not saying it isn't a cool shot, just think it's a little misleading. Feel free to correct me if I'm wrong.