Conversely, relatively un-intelligent people can get a great GPA depending on their major, class selection, and institution. Study habits also factor into GPA. I know a few people I wouldn't consider super-smart, but have great GPAs, because they study all hours of the day.
Like you said, a good GPA does take hard work and persistence, but I would also say the MCAT measures your ability to apply what you've learned, not just regurgitate (i.e. most college educations). Plus, not many people are getting lucky on the MCAT and getting great scores. I imagine a "good day" would only affect +/- a point or two.
I'm actually eerily calm about all this now. I hear that's what happens sometimes if a person is in a car crash, the world just slows down and they think "oh, so this is how I will die" and this great calmness washes over them. Haha.
I've taken two other official MCAT's and this one had (by far) the worst PS of any test I've ever taken. Thats how I know that is was the hardest test. I'm sure there are tons of people out there who have taken other MCATs and will say the exact same thing. unless they are the rainman or an astronaut or an Fe6.