I'm dead serious.
http://getrumblr.com/index.html
Could some of our legal kiwi's chip in and describe if this is a nightmare waiting to happen, legally speaking? Or is it all kosher?
It could probably be gone after for the same things irl "fight clubs" have been gone after for in the past.
Assault is the most obvious one, and consent doesn't count as a defense against many forms of it.
Arranging for acts of violence is also almost certainly some form of disorderly conduct, one of those charges they always throw at you when they're not sure quite what you're doing wrong but they're pretty sure it's wrong.
What might be the worst problem, though, is that laws regulating prizefighting and boxing would come into play. Generally sports like that only occur under regulation. A website actually organizing and profiting from it is going to step on the toes of some very wealthy, powerful people who are good at getting shit they don't like shut down.
Never mind when someone gets killed and everyone basically sues everyone else.
So if someone is going to set up such a thing, they should probably get bulletproof hosting in some jurisdiction that doesn't give a shit, because any of their people in the U.S. or any non-shithole country are going to be at serious risk of lawsuit or arrest.