Louis Wu has reported over at HBO that the recent exposure of the Halo launch party on some of the larger Xbox fan sites has generated a deluge of RSVP responses; about three times the number they can actually withstand. Matt Soell has also posted in the forum about how they will handle the situation:

We're building a list of attendees in order of when they RSVPed. (Thus Tom Van Sinden, the very first person to RSVP, is assured a spot.) I'll be contacting people who didn't leave names, or had some other weirdness associated with their name (i.e. I will definitely come but I might bring 27 friends, or possibly just six and that sort of thing). The first 250 people who responded will get priority; people who responded after that first batch will be admitted as circumstances allow. People who didn't RSVP at all attend at their own risk; we may not be able to accomodate you. I'll try to contact everyone who responded to let them know whether they're in the lucky first batch, so watch your email.

Matt's also asked people not to send him more email asking about whether or not someone is guaranteed a spot, as that will just slow him down trying to determine the answer to that question anyway.