Here's an example of how unpredictable the slow shutter speed panning effect can be.
Camera Maker: Canon
Camera Model: Canon EOS DIGITAL REBEL XTi
Lens: EF50mm f/1.8
Focal Length: 50.0mm
Exposure Time: 0.013 s (1/80
ISO equiv: 100
Exposure Bias: none
Metering Mode: Matrix
Exposure: shutter priority (semi-auto)
White Balance: Auto
Flash Fired: No (enforced)
Color Space: sRGB
It just happened that the wing sideboards of this sprint car appeared to be sharp and the rest of the car was blurred along with the details in the frame. That was coincidence and luck, good luck in this case, because it added to the illusion of speed and motion.
The slower the shutter speed used for this effect the more unpredictable the results, but the small number of usable frames will have an increasingly intense motion blur effect.