I went to
Pete's New Haven Style Apizza yesterday for lunch (it's on the southeast corner of 14th and Irving) and had a pretty decent white clam slice. I've had better, but for by-the-slice pizza it's quite good. I think the "Yalie" at Comet Ping Pong is a better entrant in the DC clam genre, but that's not a very convenient location.
This article available online at:
http://www.theatlantic.com/politics/archive/2008/05/petes/44774/