While GPS signals have become essential to our life, it is quite surprising that so many applications that rely on them do not have inbuilt redundancy systems should the signals suddenly disappear.
You can read a report (PDF) published the Royal Academy of Engineering and entitled "Global Navigation Space Systems: reliance and vulnerabilities".
The New Scientist published an article that highlights the dangers associated with high powered GPS jammers which are often referred to the "invisible utility" that drives everything from cars to GPS, Satellite, Apps like Foursquare and ATM.
GPS jammer or blocker are actually not that expensive and certainly cheaper than the $30 mentioned in the article; we managed to track one for less than $18 including free shipping from China.
Sold by DealExtreme, it is powered by the car's cigarette lighter and doesn't interfere with any navigation device installed on other cars because of its limited range (up to 10 meters).
The description says that it "adopts the technology of interdiction and interposition code, so will intercept the signal of satellite and break it completely" and is therefore considered as being illegal in the UK and in many other countries as well.
But as with most other RF devices, the range of the GPS jammer and blocker can be significantly increased by any capable engineer; that said, don't expect airplanes to fall down from the sky or ATM to dispense free money as fail safe alternatives exist (there are at least two for airplanes) and other non-disclosed counter strategies are already available.