Definitely get over there while you're in town. This is one of the best times to see the collection, as the crowds are small and the trees are in their winter silhouettes.
The staff just finished up an official winter display of some of the deciduous specimens
http://www.usna.usda.gov/Education/events.html Those trees will probably still be on display, as I've found some of these "events" can last longer than the posted times.
The Nat. Arb. isn't in the most convenient location in town. There is no subway stop nearby and the buses take forever to get there. Cab's a best bet if you're staying in town. If you have a car, the facility is just off of New York Ave
http://www.usna.usda.gov/Information/directions.html