http://www.youtube.com/watch?v=ZGMPBhdnMMY&feature=player_embedded
Guessing we cant do a similar event like this cause of problems with how dangerous a nerf gun can be?
Also you would need a lot of open park space, which is really limited around the Convention Center.
Technically speaking, if a con-goer wanted to host this sort of event during Fanime, they could; but not on convention center grounds. It's a potentially chaotic event and with the obvious risk for injury (in the video there's clearly people doing dives and in one bit someone threw a prop foam sword at another...), it's very unlikely that this would be okayed by staff.
Another thing to keep in mind though is public image for the con; intentionally or not, it could possibly irritate non-congoers if it's not a properly contained event. I highly doubt someone not participating would appreciate being randomly hit by a nerf dart.
It is not my decision, but considering functional nerf guns are against the 2012 weapons policy (http://2012.fanime.com/code-of-conduct/costume-and-props-rules/) I would assume that you can't do this on convention grounds.
Some area's of the 2012 weapons policy this would violate:
Quote from: Weapons Policy
What about NERF guns?
This is a common one for us. There are two rules to carry NERF:
1) It must not be functional. If you could put ammo in and shoot it, we cannot let you carry it.
2) It may be painted so long as the tip remains fluorescent.
I personally think it's better to leave a whole side unpainted, but the only requirement is the tip.
Quote from: Weapons Policy
Certain Props are banned no matter what:
...
Real weapons, projectile weapons (such as airsoft or bb/pellet guns, bows, crossbow's, and firearms)
Quote from: Weapons Policy
Gun-like props may not be pointed at anyone at any time.
Quote from: Weapons Policy
What can't be carried?
We do not allow real guns, or toy guns which can shoot things.