The FCC has ruled these devices to be ILLEGAL, and on appeal a federal court
upheld the Commission:
http://www.fcc.gov/ogc/documents/opinions/1998/rockymtn.html

Er, not quite. They ruled that ONE specific jammer was illegal...whereupon the
manufacturer put the same works in a different case, changed the model number,
and began marketing it again. The device you heard the ad for it NOT illegal,
as far as the feds are concerned. A few states ban their use.

The funniest aspect to the whole thing is that they don't work. By the time
they have sent a jamming signal to the police radar unit, the radar has already
displayed the vehicle's speed. Using the device also assumes that the cop is
stupid. If the cop sees a car traveling considerably faster than the rest of
traffic, and that car displays a speed a great deal different from the rest, he
or she is going to know that something's not right.