I absolutely would not attend the event and I would report on the invitation so as to prevent other journalists from going. Even if the invitation was from a non-crazy candidate like Obama, I'd never go.
The only way it would make sense is if you were a meteorologist or sports writer who didn't ever for any reason report on politics. Then there would be no conflict of interest. In other words, Al Roker could go but Matt Lauer could not.
Justification? McCain wants to win, and some members of the media have no problem compromising their integrity. I mean it's ribs!