Just a word of caution to some moms and dads. My older two kids are tall so they were able to ride most rides fairly young. My son was 3 yrs 1mth old when we went to disney one year and he was 41 inches without shoes so he definitely made the 40inch requirement. He did fine on Splash, Soarin, Test track, and TOT. However, if I had to do it over again, I would not have put him on Big Thunder. Even though he met the height requirement, that ride whipped his little neck around pretty good, and he is a tough little boy. Looking back, I don't think his little body was ready for that ride. He was also tall enough to do Space Mountain and Expedition Everest at just over 4 yrs, but I didn't think he was ready physically. Now, my older daughter did fine on Big Thunder when she was 3 yr 7 mths. That extra 6 mths seemed to make a difference for her (and she has always been my weaker, frailer kid).
I do think that some rides should have a height AND age requirement, or at least an age suggestion since I know that would be kind of hard to enforce.
Of course, this was only my personal experience, but I thought I might mention it to other people that have taller children and are trying to decide what rides to take their kids on. My younger daughter will be 3 yr 9 mths when we go. I havent' decided whether I'm going to put her on Big Thunder yet. She is my daredevil, fun-loving kid and will definitely WANT to go, but I'm not sure yet.