Ooo, Centreville, I'm up in Alexandria, but often drive to Falls Church for running shoes. In the DC Area (and I dont mean to have this sound like advertising) there are several options. These are just the ones I've gone to.
1-RoadRunner Sports - Falls Church VA. This is probably my favorite, very nice people, knowledgeable. If they dont have something in stock they usually have it shipped to me within a couple days. They even went out of their way to get me a set of Mizuno Wave Elixir 4's a few weeks before the Disney Marathon by having a California store ship them out here. I know they have both a treadmill for gait analysis as well as a pressure plate. I also happen to like their brand of clothing, some of their stuff is quite innovative. Its a fairly large store as well.
2-Potomac River Running - Falls Church VA (formerly Metro Run and Walk). Another nice store, very nice people. Not sure too much on what the backstory is with them changing names. They carry most major brands and have a pretty good selection of shoes. They have a treadmill for gait analysis. I used to live like right there so it was a very convenient store.
3-Potomac River Running - Reston VA. This was my first experience with running stores (I'm from upstate NY and there really werent any there). This is where I got my first Gait analysis after 5 years of running. First time I saw brands like brooks, Mizuno, Saucony. Was like night and day in terms of shoe quality and fit. Extremely nice VERY knowledgeable people, smaller store physically, but very efficiently stocked. Usually has a good selection of the higher end stuff. Everytime I have to goto Reston for a work meeting I end up spending WAY too much time (and money) in this store.
Pacers - Alexandria VA - dont shop here too often simply because its in old town, which is kind of out of my way. I like the store, they have a very good selection and pretty well stocked, especially for their size. I didnt get to talk to too many of the people myself but I have friends who rave about it.