We stopped at the Centreville, VA location around 9 PM.
We never got close enough to see their exact purpose, but there were three firetrucks flashing their lights and sounding the occasional siren for some unknown reason. Would they really need it for crowd control? I guess firemen like ice cream, too. (EDIT: Forgive me, I'm a little slow. I re-read the OP and obviously this benefits a firefighter charity so it makes sense.)
Anyway, there was a huge mob of teenagers. They must have announced it in school or broadcast it on MySpace.com.
Thanks for the heads up, OP. Wish we got there earlier.