While not a border point or a real enclave (it could be a 
juristicinal enclave), Camp David is an intresting place. While it 
exists, it is on no map that I have seen. Most people have no idea 
where it is and at best those who do only know that it is in the 
Catoctin Mountain Park near Thurmont, Maryland.
I began researching on the web about information on Camp David and 
determined that it was also know as Camp #3. I looked up the park on 
TopoZone and found that it listed all the camps including camp #3. SO 
I found the location of Camp David. It is at 39.6482N 77.4651E. I 
also tried the location on the TerraServer and found a great picture 
of the Camp. 
It is interesting to see the National Park Service map of Catoctin 
Mountain Park. It looks like they tried to carefully remove any 
reference to where Camp David is within the park but the did list the 
high point of the park (which looks like it is the heliport for Camp 
David). So their high point marker really marks the spot (of Camp 
David).
Camp David History
http://home.rose.net/~king/campdavid/
TopoZone
http://www.topozone.com/map.asp?z=18&n=4391406&e=288483&s=25
TerraServer
http://terraserver.homeadvisor.msn.com/image.asp?
S=12&T=1&X=360&Y=5489&Z=18&W=2
NPS
http://www.lib.utexas.edu/maps/national_parks/cato_95.jpg
-Andrew