Standard 19-inch and 23-inch rack-mount systems exude the sort of geekiness found in serious IT departments and data-services operations. Is it even conceivable that their days might be numbered?
Apple's choice to discontinue their Xserve line just as their own huge data center in North Carolina is about to come online might seem to suggest they think so, but chances are that new data center will be chock-full of rack-mount hardware, just mainly not Xserves.
I expect a significant contributing factor in Apple's decision to discontinue the Xserve is that they found it wasn't a competitive option for them in equipping the new Maiden, NC facility, even considering that they could sell it to themselves for something like 30% below retail.
Perhaps also, and this is pure speculation, given their investment in miniaturization, a 19-inch rack-mount system is simply too inefficient in its use of space. That might not seem relevant when the size of your data center is measured in acres, but once it fills up that space becomes precious, and configurations that waste it won't survive long. Perhaps they plan to switch to a narrower (10-inch ?) rack-mount system that can fit nearly twice as many devices into the same space. Given the size of the facility, they can probably design and build something for themselves more economically than they can buy from another supplier, especially considering the possibility of using A4 chips or similar ARM-based SOCs, paired with commodity hard drives, and running some variant of Darwin, iOS, or Mac OS X, and doing so would provide them with valuable experience, helping them gain traction with the enterprise in the future.
On the other hand, their need for such a data center may have developed too quickly for them to rely upon technology developed in-house, at least at the outset.
For most of us, the nature of the services that data center will support is more important and far more interesting, but those of us with a geekstreak will continue to wonder over the technology in use until such questions are answered.