Someone going by the initials GMP in a comment on Clarissa's blog mentioned the idea of a "threshold" of intelligence. GMP mentioned getting this concept somewhere else, so it's fourth hand by now.
Once you are over the threshold, have the basic equipment and intelligence to do research in your chosen field, then you can do it. You don't have to worry about exactly how smart you are. This is liberating because worrying about your exact iq, if such a thing is even meaningful, is pointless. As long as you are over the threshold, you will be ok. Your success will depend not on some abstract quality, intelligence, that you either have or don't, but on your management of your resources. For example, you might have a talent for writing prose that Dr. Theoretical Smartypants who intimidates you so much doesn't have. I am not as smart as Alberto Moreiras or Jo Labanyi, but it doesn't really matter all that much.
The first day of graduate school I realized that the other student accepted into the program the same year (there were only two of use) was much smarter than I was, and really, there will always be someone smarter than you wherever you go. I'm not being humble here, because I'm not a humble person, but it is pretty much a fact of life that there will be people smarter and dumber than you almost anywhere you go, in academia or elsewhere. When I was in high school there were about seven or eight kids smarter than me, and there are probably dozens smarter than me in my own university. Academics are used to being the smart guy in the room, but I can look around a room and see talents I could never dream of having, like Ken Irby's ear for verse. It is a waste of time to have doubts about yourself after a while.
(By the way, I am a much more eminent scholar than the other guy who was (is) smarter than me in grad school. Whatever happened to that guy?)
So how do you know you are over the threshold? What if you are actually not even above this imaginary line that lets you do scholarship at all? I'd say if you have a PhD from a respectable school, if you've published an article or two, if you've been engaging in the actual work in a way that's intrinsically satisfying to yourself, then you are over the threshold. In a way, blaming your lack of success after this point on intelligence is a cop-out, because it's like saying you can't get it done because of some inborn quality over which you have no control.