Steve is pretty practical as far as scaling and os issues are concerned.
Even as people where using G5's as super computers in Virginia, at Pixar which he was running at the same time; they where using custom made IA32 and IA64 machines as well as BOXXX workstations running linux and solaris.
Their site is also not run on osx, but solaris or linux or both.
They could also technically just use a number of BSDs which would be the closest to osx (or even Darwin) and they would be able to do what they need with this no muss no fuss.
There is really no real good reason for them to use any off the shelf servers since all they would have to do is call their existing oems, give them the specs and get it all made for probably less.
At this point all the necessary parts are super commoditized, and adding RAS features to x86 platforms is a relatively simple engineering feat.