-- I was on the ground in DC today too, but didn't get nearly as much (good) information as you got. I can fill in the blanks on the Old Post Office, though.
After scoping out Lincoln, Vietnam, World War I and other monuments on the west side of the Mall from about 11 am on looking for production or other clues (and finding naught), literally stumbled into a team over by the Martin Luther King Memorial at a little after noon. Two women, brunettes, wearing red fleece jackets, with film crew. They seemed to be working on some clue that had something to do with MLK. No production folks at the site, though. They talked to a memorial docent, who told them the only other MLK-related things he could think of were the MLK library (highly unlikely, it's an ugly 70's glass building in a high-traffic part of town) or an inscription on the stairs of the Lincoln Memorial where MLK gave his "I Had a Dream" speech. He was under the impression the girls would head to the Lincoln Memorial. I talked to their cab driver, who said the girls had told him not to wait as they didn't know how long they would be. That could have caused them a problem. FWIW, he really liked them, so they must not have been too assy.
A set of guys were there as well, but I only saw them from afar, as I was talking to their cab driver, who had been asked to wait. He described one of them as having longish blond hair, had no description of the other. Just then, the guys popped out at the end of the block and waved him down, so he sped away from me to get them. Saw another guy there with shortish brown hair, but didn't see them from the front. Tried to follow their cab, but they got away from me.
Went back to the Lincoln but found no teams and no production. The whole MLK thing must have been either a bad misreading of a clue or a decoy route.
Got a tip that there was a road closure over by the Smithsonian Museum of American History, so headed there. Turned out that the racers went to the nearby Old Post Office. I got there about 10 minutes after they left, no production in sight. There, they had to get their clue in a souvenir shop after having their picture taken with a cardboard Obama. The shop keeper told me that three teams came for clues, all within about 5-10 minutes of each other. All of them asked for directions to the Tidal Basin. I was told that blondes in pink were there. Unfortunately, I had to give up the chase at that point and return to life.
As an aside, the Old Post Office seemed an odd choice. Inside there's nothing but a food court and a few small, tacky shops. There's a gubbmint agency on the top floors, so you can't get in without a full security check (x ray machine for jackets and packages, metal detector to enter). Maybe CBS got that waived.
Got a couple of pics, but they're either too blurry or too far away to be of use. I'll post 'em anyway once I get them off my camera.
Sorry I couldn't get more, but it was fun to run along, and I don't expect TAR to come back to DC anytime soon. Thanks for launching me on this quest, Peach!