The law says it is legal:

http://ddot.dc.gov/sites/default/fil...de-Oct2012.pdf

Is it legal to ride between lanes?

According to the DCMR a cyclist can split lanes.

Section 1201.3(b) states:
A person operating a bicycle may
overtake and pass other vehicles
on the left or right side, staying
in the same lane as the overtaken
vehicle, or changing to a different
lane, or riding off the roadway, as
necessary to pass with safety.

Paragraph (c) in this section
states,
If a lane is partially occupied by vehicles that are
stopped, standing, or parked in that lane, a person
operating a bicycle may ride in that or in the next
adjacent lane used by vehicles proceeding in the same
direction.

I don't take advantage of legal lane-splitting often, especially if I don't think my bike will fit between the cars with loaded panniers. Most of the lane-splitting I do is in the CBD, where sidewalk-riding is illegal. Thus I do almost all my lane-splitting on Madison on my evening commute going back to VA. It's the west-bound street in the National Mall which is inside the CBD. The traffic starts to get heavy and slow near the intersection with 15th St in front of the Washington Monument. At Madison and 14th there are two lanes on Madison, a right turn lane to turn onto 14th and a lane to go straight to get to 15th. During evening rush hour, both lanes are occupied by backed-up cars. That is where I split lanes.

If I'm out of the CBD and am riding near one of those nice, wide sidewalks, I just switch to the sidewalk when traffic volume is so high that everybody is crawling.