Dennis,
Bill Clinton, in sort of a surprise move, removed Selective Availability (SA) on May 1, 2000. Selective Availability is the intentional "error" that was added to GPS satellite transmissions to dilute the accuracy of GPS devices. Military grade devices use a non-error signal, but they require frequent updates of an encryption mechanism to receive and decode this signal. Consumer grade GPS devices were subject to "wandering" up to 100 meters or more while SA was in effect, but without SA, it is more like 10 meters due to receiver components, ionosphere effects, etc. Real life, I have seen generally better than that, but not down to 2mm. Survey grade GPS devices, and those used by USGS are much more accurate, but they may employ several different methods to achive that accuracy, such as post processing data against a known position.

The Usenet group sci.geo.satellite-nav has more information than you would ever want to know about GPS and maps and mapping, etc, and when SA disappeared overnight on May 1, that was one busy newsgroup.
Bill