Slim pickings, my friend. In Georgetown, there's Rag & Bone and Barneys, though the latter has taken such a nose-dive that any joy you get from your visit will be of the schadenfreude variety.
Near U St, there's Redeem, which I've heard is pretty good, though I've never been myself. For higher end designers like Prorsum, you can check out the stores up in Friendship Heights.
Ultimately, though, the best place to shop in DC is Yoox.
whoa you are in DC? I was staying at a friend's place in capitol hill for a few days last week. should've had a little bam+g meet up