Zeke,
Interesting answer and I follow your reasoning for most of your discussion; however, I am somewhat confused by your statement that GPS
<font face="Verdana, Arial, Helvetica" size="2">actually use the earths background noise for amplification</font>
Don't you mean that the use of the pseudo-random code by GPS leads to a spread spectrum thus bringing the signal down below the background noise level? If this is the case, jamming becomes difficult because the jamming signal would either have to correlate with the long, pseudo-random code used or use extremely high power (or be very close).