This is a tough one, but I have a great idea for you. Rent the Eddie Izzard stand up DVD "Dressed to Kill" with her and gauge her reaction. He'll make it seem more normal and acceptable, which will soften the inevitable blow. Maybe you should also watch Ed Wood with her. Chances are, if you're a wild and energetic guy and the two of you talk about strange and interesting things a lot, she'll probably be ok with it. If you're a mild-mannered, "meat and potatoes" type of guy, she might be frightened. Try to make it not come as a surprise if at all possible. I would not put it in a letter.