cannabis generally does not improve intelligence. But only in heavy users, the study I read defines heavy users as people who smoke 5 joints or more per week.

The reason I think alot of people think their smarter high, is because their brains still process at the same speed, but they question those computations more precisely because their high. And so will go over them again in their head. Basically your not smarter, your just less likely to make mistakes, if you think in this way, and double check yourself.

Current and former marijuana use: preliminary findings of a longitudinal study of effects on IQ in young adults -- Fried et al. 166 (7): 887 -- Canadian Medical Association Journal