I believe Apple's server management software takes care of upgrades to machines remotely. You can just push a button and all the servers will update themselves. As for defragging, for the most part OS X takes care of that on the fly. How would it be any more of a nightmare to maintain than any other large cluster? I would be willing to bet that it would be a lot easier to maintain than most...
On the power front, the G5 Xserves drew about 400W. From the FAQ page on the VT site, it says that the whole cluster draws about 310kW per hour -- but I'm guessing that's for cooling, the networking hardware, etc, as well.
The main reason they built it was that it was cheap for the amount of processing power they were getting.
Still, this is VERY old news [
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