I find this conversation, full of generalizations and stereotypes used as evidence, to be slightly disconcerting. Excim and Realia seem to be among few that have considered both sides.
So, I'll share a story in reply to Excalibur. (background information)
I like to think I am smart. I know my friends are. A person surrounds their self with friends similar in many aspects to their self, right? Eh, guess thats the modest method of beating around the bushes. I am an intelligent American that loves to discuss politics, global warming, techno babble, and future technology such as Fusion energy and Quantum Computing. I am a geek - gogo me!
What does this point out? Simply an exception to an invalid correlation between pot smoking and intelligence. It may indicate a certain 'open mindedness' to you Excalibur, so let me digress once more.
The Valedictorian at my High School was an Honors student, with the same AP courses as myself, who graduated HS with a 4.0 GPA. I had a 3.76 GPA. She turned out to be an alcoholic, and I was not. Therefore, I shall conclude Alcohol, too, will make you smarter. I am not mocking your anology, but I am hoping to point out the fallacy.
Another girl that took the same AP courses as myself, is a fucking pot head. She did AP Calc, AP English, and AP History baked. She is an intelligent person that I would love to see have the right to smoke pot because she knows the difference between being impaired or in full control of her faculties. I have no problem with people smoking pot so long as they realize its effects have empirically proven effects on reaction speeds. So what does this mean? First, I just want to point out the phrase 'empirically proven.' I may not trust the government to say it is bad, but when verifiable sources prove Marijuana's effects, I won't argue the case against that evidence. Second, I think this shows I am open minded in my belief that people should be able to do what they want, so long as it is only harmful to their own self.
Then, in an [(edit: almost)] final remark, I had the "benefit" of going to highschool at a Tennessee public school. Every school has their class clowns, but Tennessee has its share of class rednecks. Those pot users are stupid. They are not open minded, they are not intelligent, and likely pot serves as a gateway drug to Meth. By your argument, they should be respectable, knowledgeable, tolerant people. You should be able to carry on polite conversations regarding gay marriage, gun control laws, and abortion.
And forgive me, because this does come across as mocking the idea. The day you can have that conversation with the latter group, without discrimination against gays, a reasonable, logically based, DEBATE on gun control laws, and thoughts that comprehend the full responsibilities and scope of an abortion, not just what was told to them or read in the bible -- then, I will be more apt to believe that pot makes you a more tolerant, intelligent person.
Final Assertion:
You may be attempting to make the generalization that people inclined to start smoking are already intelligent/tolerant people. In which case, I would also say there is some fault to be found. I have a few friends that smoke pot currently, that are very tolerant intelligent people. I will only believe that boost to intelligence/tolerance is because of the pot if science yields results to that effect. In which case, I will be the number one proponent of jacking up the country's rednecks on so much pot they'll have permanent red eye -- not just what you see in a photograph.
EDIT: And oh yea... I don't smoke pot, but if you did read this full post, I hope I came across open minded.