seacue: Should you by any chance have to go to the main terminal for Customs etc, you would be herded into special mobile lounges.
The info you've given is, IIRC, right for UA. But for anyone else going to IAD, there are still plenty of airlines where you're taken off the aircraft by mobile lounge and taken to the main terminal immigration facility.
If that happens to you, trick number one is this: Be the last off the aircraft. Do anything you can to be last off the aircraft. Because the driver sits at the end right up against the aircraft, and the last person off the aircraft onto the lounge is the first person into the immigration queue.
I get no end of strange looks from people filing past me as they stampede to get off the aircraft at IAD. 10 minutes later, they understand.
