Thought this was pretty interesting - PC case that is designed for open air use. Would be good for people changing their hardware every other day.
I don't think dust is a huge issue after opening some of my cases after years of use to see the computer running fine just about knee-deep in dust. Probably give it a blow of with some compressed air every now and then just to be safe.

Here's a modded one:

They would take up a lot of space and would be pretty awkward on a desk compared to a tower though. I guess you would want that kind of access to them if you are changing parts constantly.
The Antec equiv is a bit different:

The Lian Mini-ITX spider case is different again:

Here it is with some hardware mounted:

What do you reckon?