Getting out there is not such a problem... metro to West Falls Church or Vienna and you can take a Fairfax Connector (or GMU Shuttle) there. By the time you get to the end of the orange line there's usually a big enough crowd of obvious concert goers you can fall in with them.
Getting back though is a problem. Your best bet is hitching a ride in the parking lot. Even calling a taxi is useless, unless you arrange before the show for one to pick you up afterward.
A few years ago at a Beck show there, my friend (who had a cast on his leg) and I tried for an hour to get a taxi, but every time one showed up, someone else would take it. So, we hobbled a mile down one of the Fairfax Co. roads into a strip mall where there was some Chilis-type Tex Mex bar restaurant and called a cab from there. By the time the cab showed up -- another hour later -- the bar had closed and we had to sit on the curb. Also, during the hike over to the bar, we had picked up two other strangers (one of which was an exchange student from Argentina who spoke very poor English and was trying to get to Rockville, MD, while we were going to Adams Morgan).
An interesting night, but one that made me avoid the Patriot Center until I got a car of my own.
