I can't remember how we found this guy but he's local, not a big restaurant, and did a great job for my son's 1st birthday.
http://www.bigmooksbbq.com/

I liked using someone local (he's actually in my neighborhood, Pimmit Hills), and I think the prices are better than going through a restaurant.

Willard's in Chantilly - not close in but they have some great catering packages and their pork/beef is excellent.

Another vote for Dixie Bones (they catered a party for us a handful of years ago), but I also liked Willards (they catered a party for my friend just last year). Both are in Northern Virginia.

Of course, the best barbecue catering ever (and those of you from eastern NC know whereof I speak) is Bill Ellis of Wilson, NC (http://www.bills-bbq.com/). And yes, I've seen their catering truck up in this area before. But I imagine it's pretty damn expensive to get them this far north. Too, too bad....

Lazy Pig in Dumfries. I don't know if they deliver, but my god - the sides, the meat...

Am I the only person who thinks Rockland is awful? I've only had the one in Arlington, so maybe it's better at other locations?

Red, Hot & Blue tastes like they use liquid smoke.

I love the Rocklands in Virginia Square and have eaten just about everything on their menu, but have never ordered catering from them. Still think it's a safe bet, though.