As a DC area coder interested in startups, I thought I would map this to see where the hot spots are. I was not surprised to learn that there were several along the Dulles Toll road, but I didn't know that downtown DC was a favorite.
I followed the links to each startup's site. If they didn't provide an address on the site, I checked their whois. If that was private, then I didn't add them to the map. So there are definitely a few that didn't make the cut because they're being extra stealthy.
My map: http://bit.ly/fO9gyL