In further confirmation of chfenton's post:

Yesterday, I sent an e-mail to the airport office asking whether the Concourse D line was for "true" first class ticketed passengers only, or whether elite passengers were also allowed.

To be sure my meaning of "elite" was clear, I specifically included in my message the exact wording "American Airlines AAdvantage Platinum" and "United Airlines Mileage Plus Premier Executive" (figuring that the middle tiers would be the most representative).

I received a very nice reply confirming e-mail today stating that the line IS "meant for the premier customers of the two airlines that you describe". The message further explained that the wording of the sign above the line threshold was debated and carefully considered, and that the wording "First Class" was chosen as a catch-all compromise.

Notwithstanding this welcome confirmation, if an AA or UA agent offers to stamp my BP, I will certainly accept it - you can *never* have too much insurance