Depends on how you jam it. I'm tlkaing in military terms here, but the same basic principles apply. The idea of jammers is to keep a radar from locking on to you. There are two types, a Noise Jammer and a Deceptive Jammer. Noise jammers simply overpower the radar and flood the frequency it operates on with white noise. Deceptive jammers try to confuse the radar and make it think the target is somewhere it isn't. i don't know how effective a deceptive type jammer would be on a speed calculating radar since it isn't trying to find your location. But a noise jammer should block them. If successful they would get anomalous junk readings, most cops would think the radar is broken.
Another cool military technology that they actually use in cars is called the RWR or Radar Warning Receiver. It detects incoming radar waves and sets off an audible alarm. In combat aircraft it actually has a little screen to show you what direction it is coming from.
EDIT:
I did some reading, apparently it's a felony in the US to have a radar jammer in your car.