I don't think any 39" child is subject to death and dismemberment on Soarin', or Splash Mountain, or Test Track...ok, maybe Test Track since we didn't bring our car seat with us.

(Though we typically don't use it in the car, either. Two phonebooks and a shoulder strap are FINE...car seats, pfft...just The Man tryin' shake down hard workin' parents for more money!)
That said, on our most recent trip our youngest measured up to 40 1/2" at home against the wall with no shoes. We figured no problems once we got there...WRONG. Apparently inches in Ohio are slightly smaller than those in FLA as we went to get on Soarin' and got the Nazi-Measurement lady who was sliding a fingernail between my son's head and the bar. No riding Soarin'. (Though he'd passed Test Track already, so consistency isn't high on the measuring thing.)
No problem, rules are rules, don't throw a fit. Instead, walk outside, get four park maps, fold two each into said son's shoes, and Voila! he's on everything. He got measured plenty, and by the end of the week would just walk up, hit his head on the bar, and smile at the CM.
Was that 16th of an inch the difference between life and death? No. Was it bending the rules? Oh yeah!

But here's my lame rationale: Cost as much to get him in as it did my older son who can ride everything. (Told you it was lame...yet financially responsible.

)