Your talking about [b]SA, Selective Availability[/b], a process by which the military can switch and shift the positional readings of GPS units. There are different levels up to 300 meters if I recall correctly.
Early on when the DOD used SA, corrections had to be provided to planes, ships and survey/ engineering companies (which I work for). Mr. Paul is correct and the DOD is no longer allowed to make use of this feature.
As a general rule Trimble makes most of the GPS equipment used by the military and Scouts and Explorers are some of the finest handheld units available on the market. All our survey units are Trimbles and survey units are capable of sub-centimeter accuracy. Top line hand-held units will, with a 6-9 satelite hook up, produce about 3-5 meter accuracy and run from 300-800 dollars.
FYI
Mike