+1 for Som being awesome. Haven't been there on RSD though.
If I was doing any DC area store for RSD, it would be CD Cellar in Falls Church. But that's mainly because it's my favorite record store to begin with, and there's not that much I'm interested in for Record Store Day. I do know they're participating, but they couldn't tell me what they'd be getting. They're always pretty busy on Saturdays, so I imagine that'll only be worse on Record Store Day.