The windows client Google Talk client does not have a feature for invisibility yet. So, if you ever want to sign in as invisible, you can make use of the web based Google talk which comes along with Gmail.
Log in to Gmail and on the left you can find a link which enables you to sign in into Chat. After signing in, you can set yourself as invisible. When you are in the invisible mode, you wont appear online on your friend’s Google talk friends list.