Hmmm, I've always found that my ID gets me served at the PX on US bases, although there was this one cashier at a base in California who was very confused by the whole concept of a non-US miltary...

Actually, it has proved useful - it got us through immigration in about a quarter of the normal time as it meant we were told to stand in the fast-moving 'US citizens' queue instead of the long and slow 'dubious foreigners' line.

MC