Pine Needles blogged about these incredibly cool pens today called FriXion pens. I was trying to get around to blog about them myself, because I think they are that cool. But they beat me to it, so I'll just link instead(that's a lot easier!)
They have a video on their site that shows you how they will disappear with the heat of your iron! You can also purchase some at Pine Needles or through their website. Go check it out HERE.