That's a good question. Limited space and the two-session time-frame are the constraints, but you're correct in pointing out that new folks coming in might not find open slots for getting their gamin' freak on.
We've got a pretty kick-ass variety so far, with regard to multiple settings and game systems:
Deadlands, HEX, GrimTales, Dread, Spycraft, Delta Green, Spirit of the Century, and now
Shock -- and several of these are already booked out. Hell, I want to clone myself so I can be at just about every one of these!
In re: new folks, I wasn't sure if we were still keeping this low-key or not with regard to promoting. I posted a link to the DC Gameday website in my DC indie gaming LiveJournal community, read by a total of FOUR folks (look out, EN World!

), and that brought in Daniel (dikaiosunh) and might bring in one or two others. I also mentioned the event to Fred Hicks (evilhat, SotC author) just after GenCon, and I assume either he or you folks got the word out to his partner-in-crime Rob Donoghue (rdonoghue, another SotC author, who registered last night).
Therefore, I'm willing to bet we'll see a small rush of folks trickle in over the next few days -- and I apologize if I jumped the gun on that.
My experience shows that there will probably be no-shows on the day itself as well as last-minute walk-ins ("hey, I brought my friend who I just told about this last night... is it ok if...?") -- usually these will serendipitously balance each other out. But if more folks will likely be registering, we have three sessions open as of today, and if the new folks aren't interested in
Spycraft, Delta Green, or Spirit of the Century, they're out of luck.
So yeah, if we can find folks to run stuff, we could probably use some more GMs with cool events on the schedule. I can definitely scare up another person to run a HEX event, and possibly one other to run something indie -- I'll wait to hear what everyone else has to say before I cause the body count here to increase any further...
--tom