Here's some good back and forth on this.

http://www.answers.com/topic/9-11-conspiracy-theories

Especially Michael Schermer from Scientific American:

http://www.sciam.com/article.cfm?art...1583414B7F0000

and the Popular Mechanics article:

http://www.popularmechanics.com/scie...tml?page=1&c=y

And an answer to the Pop Mech articles

http://911research.wtc7.net/essays/pm/index.html

I haven't looked into this, but my "knee-jerk" is that theories claiming 9/11 are a hoax can be debunked in the same way that anti-evolutionist claims are.

However, I'm going to look at this, and see how strong the evidence is.