To make it stand out. That's pretty much it.

We wouldn't be talking about it right now if it wasn't some weird black cylinder.

Say what you want about Apple, but they definitely know how to market.

A cylinder is a very silly shape for a computer IMO but that's just the sort of thing Apple does.

They might even have brainstorming sessions that go,

    "Ok what would our competitors *NEVER* do. Yes Dave?"

    "Make a cylindrical computer!"

    "Brilliant!"

I'm waiting until the next MacPro looks like the monolith from 2001 A Space Odyssey. The entire grey black slab could slowly pulse a über faint glow like the power light on late model PowerBooks. The idea would be to eliminate all indicator lights and make the entire monolith surface into the indicator that it's on and replace brightness with area.

I present to you, your next MacPro:

http://atypicalsnowman.files.wordpress.com/2008/03/2001_mono...

For what it's worth, on the Mac Pro product page, there's a snippet about a "Unified thermal core" with a graphic showing a birds-eye view of the machine that would lead me to believe the cylindrical design has an actual purpose to it.

The thermal design is pretty straightforward. It was used in some supercomputers, as I know.

But, why the "chassis" is a cylinder? They can use the same shape as the G4 cube, seriously.

It doesn't need to be, but the fan is circular, making it a slightly more efficient shape. They probably made the first one aluminum color and thought it looked like a trash can, hence the black color.