This New Scientist article goes on about the effects of disruption of the GPS network. It is precisely because of this large potential for disruption that Lightsquared will not be permitted to operate their network if it interferes with GPS.
I am entirely unsurprised, BTW, by this quote from the article:
"We originally expected that jammers might be assembled by spotty youths in their bedrooms," says Last. "But now they're made in factories in China."
This is one of the areas where the Chinese absolutely wish to “encourage” development, as GPS jammers have an important dual-use for them (degrading the American tech advantage if the Mainlander Chinese ever actively invade Taiwan).
As the state of the art progresses, the parameters of the problem will change, I’m sure.