OK , this might be a bit controversial, and I don't think I'm a conspiracy nut, but I do think there was a conspiracy of sorts to get the USA into WW2.
I think there are several things that are beyond doubt:
1) FDR wanted into the war because he believed that the Germans were a potential threat to the USA.
2) He knew he couldn't get a declaration of war through Congress.
3) He had tried provoking the Germans in the N. Atlantic, but they wouldn't bite.
4) The US in negotiations with the Japanese presented them with humiliating terms, which there was no chance of them accepting.
4) When the Japanese rejected the terms and the embargo was imposed, FDR knew very well that it would lead to war.
6) The US military was expecting war with Japan (there was a 'general war warning' sent on 27th November, so the attack was very far from being a 'bolt from the blue'.)
5) The US knew that Japan was going to attack US interests on the 7th Dec because the declaration of war was intercepted and decoded the day before before it was presented by the Japanese ambassador.
Does the above amount to a 'conspiracy' though or is it just normal politics and foreign policy? Some people feel that if FDR knew about an attack and allowed it to happen without allowing the forces to take appropriate defensive measures that that's going beyond normal politics. So the question is did he know anything specific, and if so were commanders ordered not to prepare or was it just incompetence
The problem is that there
was evidence available to the US that Pearl Harbor was going to be attacked, it's impossible to say whether it was deliberately ignored or people just failed to put 2 and 2 together.
Personally, I think attacks were expected on the Philippines rather than Pearl Harbor.
As for the effect of the 'conspiracy' (if there was one) being exposed, IMO you would get a situation similar to the one in the USA over Iraq now, e.g. a massive divide between the people who believe the president is telling the truth and the people who don't. The 'proof' of a conspiracy would be unlikely to be cut and dried.
And you still get the reality that the country is at war, even if the President has possibly lied the country into it; and with it being a far more serious war with mass mobilisation etc there's not much the average person can do. I think they would knuckle down quite quickly.
There's a fairly balanced, but a bit of a mess, article on the subject on wikipedia:
http://en.wikipedia.org/wiki/Pearl_Harbor_advance-knowledge_debate