I commute into DC every day on a cyclocross bike. Good choice for our area, given the erratic road surfaces, plus the ability to ride the same bike on the canal path and other non-paved areas.

My favorite route from near Silver Spring is via Capital Crescent Trail (18 miles each way from my house to my office), which includes 3 miles of gravel at the start, plus several uninterrupted miles of road-bike-worthy paved portions, followed by car-infested city streets. Cyclocross bikes strike a pretty good balance for all of these situations.

A number of the local bike stores sell cyclocross bikes at fair prices. For example, City Bikes in Adams Morgan has a good selection. I ended up buying a Bianchi Volpe on eBay and building it up myself, but I probably would have saved money and lots of agony had I bought a new Surly Cross Check at City Bikes instead, which I nearly did:
http://citybikes.com/product-list/cyclocross-all-pg59/

I think City Bikes and a number of the other local stores will give you a reasonably balanced view of their products versus other options. For example, I really like Bicycle Place in Silver Spring, and also The Bike Rack on Q Street and 14th. They are very supportive of the local biking community and in it for the long haul. Good luck!