Have the rules changed in the last year? I did exactly that ten months ago...booked a cheapie BWI-ORD ticket (thanks to Southwest effect no doubt), but called UA when I was stuck in Northern Virginia traffic to see if I could stand by out of DCA or IAD. The phone agent asked me "which airport are you closest to?" I said Dulles, and he told me "sure, there's this thing called the co-terminal rule. Go to the ticket counter at Dulles and they'll put you on standby for the next flight to Chicago."

Sure enough, the IAD ticket agent gave me a DM card for the ORD flight. I was SSSSed at security, which was annoying, but eventually made it onto the wide-open IAD-ORD departure in E+.

I've also flown standby back to BWI on an LAX-IAD T-fare ticket, also with no problem.

US Airways is much more strict about standby, in my experience; on US, you can only standby to or from a coterminal if the same exact fare is offered from both airports. Many discount fares on US, especially in the pre-FlyI days, were only valid from BWI - a pain for this Virginia resident.