"If we can jam it, so can folks that won't tell us about it first." Don't worry too much about GPS jamming. One of the hacker mags ran a bit about how easy it was to build a small jammer and launch it on a balloon, etc. One of the replies soon after was from someone in the military who said they'd enjoy the opportunity to have that problem. If the jammer is on the ground--it is easily RDF'd and terminated. And if the jammer is in a relatively unpopulated area? It is RDF'd and terminated by an antiradiation missile (ARM) available for that purpose. If it happens to be with the bad guys while it is operating...they get shut down too.
GPS jamming can be an inconvenience, but isn't expected to be much of a problem. Active jammers just make too good a target.