Here is an interesting take on the PC case, The Acrylic Cowboy PC Case.
The Acrylic Cowboy PC Case is an open PC case, can it actually be called a case? I am not sure, anyway it is designed to hold your motherboard, PSU, and DVD drives.
The Acrylic Cowboy PC Case will take ATX and Mico ATX motherboards, and all your components are completely exposed, it definetly would keep everything nice and cool, I am not sure how good it would be though as you would probably spend most of your time removing dust from it.
If you want one they are available for $75 from GeekStuff4U.