I didn't get the impression there was any cost involved here. They told me
that they look over the list of public interest events that are going on in
Washington, and the night before, they decide which event to tape/broadcast.
They don't give advanced warning that they are going to show up; you just
let them know the event is occuring. If it turns out that socks, the
whitehouse cat, has kittens, they probably won't show up 
The compelling reason to let CSPAN know about the NANOG is that the
broadcast quality is likely to be *far* better than the MBone. We are in
Washington and someone is willing to broadcast it for free. This probably
won't happen again for a while.
Bill