The body of knowledge about GPS jamming is growing quite a bit thanks to the uptick in countries that see themselves as badasses:
1) Jamming is most often directed at the L1 signal, but L2 is sometimes hit as well.
2) Jammers (personal privacy devices and high power military transmitters) use strong "RF chirps" that sweep the whole band and cover up any weak GNSS signals.
3) Actually spoofing a GNSS signal to cause a gross navigation error is difficult, but not impossible, and must be directed at a specific victim. Very expensive - but motivated governments would do it for a valuable target.
4) Aircraft will experience an increase in ANP and have to fall back on DME-DME as the next most accurate navigation method.
Here's a good place to troll for technical info on the subject:
Massive GPS Jamming Attack by North Korea | GPS World
and
GPS World - April 2012
Look to the Russians and Chinese if you want to buy this technology. Cheap devices can cost a hundred USD; high power jammers may run several thousand USD and up.