Hey,
I also live in DC and just finished up my own build. It's the Gary Fisher Marlin on the Mountain Bike page. I've talked to a lot of different police officers around the city about the legality of what I have. Not surprisingly, everyone seems to have a different opinion. No issues (tickets) so far. I figure as long as I obey traffic laws no problem. The issue I face is that anything with an engine in DC needs to be registered.
Upon trying to register the bike, the DMV laughed at me because I brought them a bike. However after further review of the "non-traditional motorized vehicle chart" (upon my own request) they said take the engine off and stop being so creative. Apparently you cannot register a motorized bicycle without a title. Having built the bike myself, there obviously is no title. I have also spoken with a friend who is a prosecutor for motor vehicle violations, and she made it seem as though no registration was required.
Is it a bike, motorized bicycle, scooter, moped, or motorcycle? In some way, it fits in every category. Whatever it is called, it sure is a blast to ride.
The laws are extremely gray...