ATX motherboard?

I would say it is. But how are you going to get the FB-DIMMs cards into a rackmount case? I thought I heard somewhere that the motherboard is made by Foxconn so I'm guessing it would fit an ATX case.

I would think the Mac Pro is a bit new for anyone to have tried a stunt like that. Without knowing the physical differences you may want to pose this question in the PowerMac G5 forums. I would think they would be similar enough to maybe get a "near enough" answer.

You could always try it and tell us. 😉

There is a company that does a rack-mount modification for G5 cases, I suspect the same mod would be doable on a Mac pro.

http://cgi.ebay.com/PowerMac-G5-Rackmount-Modification_W0QQitemZ5881140982QQihZ0 08QQcategoryZ80032QQssPageNameZWD1VQQrdZ1QQcmdZViewItem

As ATX is a desktop form factor and the Mac pro eventhough a "desktop" has more in common with server form factors, I dfoubt it is fully or quasi ATX

No.

It isn't an ATX motherboard. It is, quite possibly, the farthest thing away from the ATX standard that you could hope for.

The hole mountings are different, and the Powersource isn't ATX. And you've got the FBDIMM risers that need support from the case chassis so they don't damage the mainboard connectors.

Even if you did manage to somehow put it in a seriously hacked up ATX case, it would have to be probably more then 5U tall, since the Mac Pro has the large tower-like heatsinks and again, the FBDIMM risers.

It would probably be easier to just get a flat rackmount support and sit the Mac Pro ontop of that, in the rack. Yeah, it takes up a lot of room, but there isn't any other way to do it.

-S'Captain