But what about football games? When you watch a football game on TV, and the camera shows all the people in the stands, do you think the TV people got permission from each and every person in the stands to photograph them? Of course not. Why? Because they're in a public place. In the United States, the law is pretty specific. It's legal to take pictures anywhere except on certain military installations or in places where a person has a reasonable expectation of privacy (such as your bedroom or shower). That's it. Basically, except for those instances specified by the law, it's legal to photograph anyone, with or without their consent, even if it's on private property. Out in the open in a public event, such as Burning Man, there is little reasonable expectation of privacy.
Publishing and making money off of photographs is a whole other matter, but just taking pictures for your own use is completely legal. Even if you don't "allow" pictures to be taken of you, that doesn't make it illegal. In fact, if you go after someone who has taken your picture, you run the risk of being arrested for harassment or coercion.
Bottom line: if you're at a public event --like Burning Man-- you're gonna get your picture taken. Get over it.
Article on photographer's rights: http://www.usatoday.com/tech/columnist/andrewkantor/2006-08-11-photography-rights_x.htm
Downloadable PDF document of photographer's rights: http://www.krages.com/phoright.htm