FROM FCC -- "The FCC considers the use of traffic radar jammers as malicious interference and strictly prohibited by the Communications Act of 1934, as amended, as well as by FCC rules. Anyone using a jammer risks such penalties as losing FCC licenses, paying a fine, or facing criminal prosecution."
Note the FCC does not have jurisdiction over laser jammers, only microwave jammers (really all RF and microwave sources).
Starting July 01, 2006 it is illegal in Tennessee to use a radar jammer. Violators are subject to citations and confiscation of the jammer.
Illinois just passed a law making jammers illegal. The law also states police cannot stop someone solely based on suspecting they have a jammer. However if stopped for some other reason and a jammer is found, the fine is $50 for a first offense.
Most police are not too concerned about jammers because most jammers do not work. Other legitimate transmitters (and unintentional radiators) can be more of a problem in certain locations or situations.
Some laser jammers are effective sometimes, but usually only at long ranges (around 1000 feet). Jammers that do effect radar sometimes give themselves away (speed readings change rapidly and by several tens of mph). Laser jammers are not illegal in most states (except IL and TN). This will probably change as (if) laser jammers improve.
CopRadar