Looks like the US DOT (FAA, Coast Guard, etc) are finally realizing that ground based navaids should be left intact and maintained because of the relative ease to degrade and/or scramble the GPS signal. Was at a boat show not long ago speaking with a Raytheon sales/engineering type, and they had received a number of complaints from boaters in the Tampa Bay area (Florida, USA). Seems the D.O.D. had affected the signal near MacDill AFB, home of the US Central Command, which was/is controlling the effort in Afghanistan.
As to the accuracy of Differential GPS, I remember reading of a NASA project to drive an attitude indicator in a general aviation aircraft using DGPS. As I recall, the differential transmitter was in the center of the aircraft with receivers at each extremity – the initial test flight resulted in a slightly jumpy display of the attitude display. After downloading all the data, the culprit was found…..wing flex.