Poll Vault: Angus-Reid
According to this, 26 percent of Americans believe that the 9-11 attacks were a pretext for war:
In the online survey of a representative national sample of 1,007 American adults, 62 per cent of respondents disagree with Ahmadinejad’s claims that the “Sept. 11 incident was a big fabrication as a pretext for the campaign against terrorism and a prelude for staging an invasion against Afghanistan.” Only 26 per cent of respondents agree with this notion, and 12 per cent are undecided.
However, note a few key points here. First, it's an online survey, not a telephone poll. Second, in looking at the actual question asked, it did not mention Ahmadinejad himself, it just as if they agreed with the statement. Third, another question asks, "The commission that investigated the attacks of September 11, 2001 concluded that an attack was carried out by 19 hijackers who were members of the al-Qaeda terrorist organization, led by Osama bin Laden. Do you believe this is actually what happened on 9/11?" For that question, only 12% said no. In addition, in the crosstabs, we see that while 33% of self-identified Democrats agreed with Ahmadinejad's statement, only 13% disagreed with the 9-11 Commission, although there is a large number of "not sure" responses (20%).
When we go to some of the specific conspiracy theories, only 15% find the controlled demolition of the WTC credible (with 11% not sure and 74% dismissing it). The shoot-down of Flight 93 is also found credible by 15% (with 22% not sure and 62% finding it not credible). No plane at the Pentagon checks in at 13% credible, 11% not sure and 76% not credible; those numbers are virtually identical to the controlled demolition numbers. TV fakery? 6% find it credible, 7% not sure, and 87% find it not credible. Unfortunately, there are no crosstabs to let us know the percentage of Democrats versus Republicans on those questions.
Overall, I'd say that the results make it pretty clear that somewhere between a quarter and an eighth of all Americans are Troofers. But, as I have said often in the past, just because (say) 30% of all Americans believe Elvis is still alive, doesn't mean they spend their spare time searching for him. Active 9-11 Truthers are probably smaller than 0.01% of the population.