Have you opened up a MacPro tower? It's a thing of beauty. It's airflow is well thought out, one of the first production machines to use liquid cooling. But the best part is servicing the machine, everything is very minimalist and easy to pop in and out.
What about a closed version of the Antec Skeleton with something to pipe heat out of the top?
-Zach
MacPro has always been air cooled as far as I know.
PowerMac G5 was watercooled... (case looks the same)