I tested a 10.7.3 machine that is running iTunes 10.6 and the checkbox is greyed out. (for both iOS 5.0.1 and 5.1)
I tested a 10.7.3 machine that is running iTunes 10.5.3 and the checkbox is greyed out. (for both iOS 5.0.1 and 5.1)
I tested a 10.6.8 machine that is running iTunes 10.6 and the checkbox is not greyed out. (for both iOS 5.0.1 and 5.1)
I am going to say that this is a OS problem.
Strange though, because it used to work with our first 10.7 machine that was running iTunes 10.4.x. We've recently upgraded that machine to 10.6 and now can't sync mail accounts with it either.