I have to admit there are not too many people with desktops. At least I don't remember seeing too many of them. I have seen a few really nice ones that look more or less like dvd players. Nice for media centers.
Heh, I got one I hacked out the back of to fit an ATX board in. It's not in use at present. The drive bays interfere with the heatsink/fan for most boards.
Easier to mod for modern boards would probably be those old IBM AT 286 style desktops, due to them being wider. Then you don't have to have the mobo running under the PSU, Also you get more front panel and back panel for fan mounting. If you get a big one, you've got 6 5.25 drive basy to play with. Though the bottom ones are usually at least half internal unless you hack up the front panel. But that's probably where you'll wanna put HDDs.
Thinking about it, those kind of cases should actually be very nice to cool, since it would be easy to lay down the drive cables etc, out of the way on the bottom of the case, and they will have a chance of staying there.
They also had hulking great PSUs in them, so if you want to put dual PSUs in, one for the drives and one for the mobo, that should be easy.