I just bought my new road bike from Cycle Life. They do sell a Soma frame, but I would not buy or bring in a touring bike for a long-distance self-guided tour from them. Here was my two recent conversations there:
"I see you sell SOMA frames. That comes highly recommended." Store Rep 1: "Yes, it's a great bike. We can set you up with 32 spoke wheels. Wouldn't that be great?"
Store Rep 2 (subsequent stop at the shop). "Why would you want to buy a steel frame like a Soma? It's too heavy. Why not get a carbon cycle-cross bike and use a Bob Yak. Next fall, there will be a whole slew of carbon cross-country bikes being sold that come with disc brakes."
Again, I really like Cycle Life. I would not give them my business if I thought otherwise. I also like City Bikes and The Bicycle Place and several other DC LBS mentioned in a prior message. I am not sure however if I would rely on any of them to do a last minute tune-up on a touring bike before a long distance tour. At a minimum, I would have detailed telephone conversations with the mechanics at the shops before arriving in DC. Note: depending on when you arrive in DC, you may need to schedule far in advance to get a mechanic to tune up your bike.
Finally, I also was going to recommend REI like another poster did, but was going to suggest the shop in Rockville since it's much easier to get to from Bethesda. However, I had never used REI before for bicycling purchases/repairs so I could not vouch for what to expect. I'd definitely make a call to the two shops in Rockville and Silver Spring, especially if the other LBS mentioned don't pan out.