Space Mountain is listed on the official Disney site as 44 inches.
If you go to the
DIS site height/restrictions calculator, and put in 40 inches as your child's height, Space Mountain comes up as having a minimum of 44 inches. Splash Mountain and TOT are 40 inches.
One of the new things we noticed on our trip in March 2004, was that most rides/attractions have new signing at the entrance. The signs give a short (very short) description of the ride (things like "Turbulent Thrill Ride), and an icon that indicates whether someone with a wheelchair needs to tranfer into a ride car or may stay in their wheelchair/
ecv for the entire ride.
The signs also list any ride restrictions, including height and they include a permanently mounted "You must be this tall the ride" measuring stick. I don't have pictures on this computer, but will try to find where I put them on another computer so I can post some.
There were several rides where some of our party went on and I waited at the entrance. At each of those, a CM was stationed and was directing which kids could go into line, based on whether or not the met the height requirement. Since the wait was fairly short, DH, niece and nephew went on Triceraptop Spin (with a 48 inch minimum) about 4 times in a row, so I had lots of time to observe some interesting things there. I saw people who tried to sneak a too short child in by "hiding" him between 2 adults. I saw a lot of people trying to stretch their kids (by having them on tiptoes or tilt their heads back) or wearing a tall hat (which the CMs made them remove for measuring). One whose kid were short by 3 or 4 inches said that the child was tall enough at the doctor's office and one guy who insisted very loudly that his child had ridden yesterday (when the child could not even touch the top of the stick with both hands on top of his head). I also saw one child who as tall enough, but kept bending to walk under the stick. It took a while for them to get him to just stand up and be tall enough. I saw one boy (just a tad short of riding) whose family said he had been tall enough to ride before and then the boy remembered he had worn different shoes the other time he rode. The CM was nice to him and told him to make sure he wore the other shoes the next time he rode.
So, although a lot of people are being truthful, there are enough who are not to make the CMs sceptical. Not an excuse for the CMs to be rude.