Um metal is kinda defined by distorted electric guitar so no there is no such thing nor will there ever be all-acoustic metal. Are you asking for metal with a lot of acoustic passages maybe?
Agalloch is the closest I can think of to what you're talking about. I don't even know if you can properly classify a band as metal with accoustics being dominant.
Haha. I have an original one of these somewhere Seth drew the art for. You should have saved this reply for the Metal Where Someone Beats On Their Knees thread.