Originally Posted by John Clark
You will only be charged if your phone rings while you're overseas and it autoforwards. If your phone is off you won't be charged. Having your phone off isn't the greatest idea, though!
I was told (and read online) that calls will automatically be forwarded to your voicemail, regardless of whether the phone is off, you "reject" the call, or just don't answer. In either case you will be charged.
To be sure I opted to have VM disabled. Also because, as you already mentioned, it isn't the greatest idea to have the phone off the entire time.