This happened to one of my guildies tonight.
While we were chatting about random things in guild chati she suddenly said that a GM was talking to her and that she was reported for scamming people. She said that then the GM was asking for her password to make sure that the account really belonged to her. We told her not to give out her password and that a Blizzard employee would never ever ask for her account details. Sadly she said that she had already given her CD key, e-mail address and username. Then we advised her to change her passwords into something more complicated and change her password questions as well. And also contact the costumer support of course.
So please.. Under no circumstances, do not give out your account details to anyone no matter if he has a GM tag in your chat log or not.