I've never charged a shuffle on my mac (my wife uses our PC for her itunes), and we have this problem often. I have to completely log out of my XP profile so that my itunes over there won't start up. Then, she has to quit itunes completely on her XP profile and then plug the shuffle in. The shuffle then launches itunes, and it will start charging.
Sometime, even when it is not "enabled for disk use" the shuffle will still act as if it is...very annoying.