When you land in the US and bring in alcohol, US Customs says you're supposed to be over 21 (even though that's state law).

Does US Customs enforce other state liquor importation and tax laws?

For example, will a DC (or VA, etc.) resident be arrested if they land at BWI with a bottle of alcohol? [Since it's a felony for anyone other than an MD resident to enter MD with a bottle of booze, or for an MD resident to bring in more than one liter, IIRC. This law was passed 6 years ago.] I assume in practice the answer is no, but could a Customs officer choose to enforce this kind of state law if he/she wanted to?

I found this related thread, which seems to indicate that non MD residents better not bring booze into BWI http://flyertalk.com/forum/showthread.php?t=397819