With the arrival of "Internet search engines" like Google, Yahoo or MSN, it is now a common practice to administer a pretty large cluster of machines. It is not uncommon to find these days clusters that comprise hundreds of thousands of computers. So it is not that hard to make a step from here to, say, clusters containing millions (or dozens, if not hundred of millions) of machines. 

But there is a little problem here - when you have clusters at this size, machine failures will become noticeable. Especially when you use cheap hardware, which BTW makes sense given that you have to keep the cost down with these gargantuan clusters. And this problem generates another problem - who will do the wiring, replacing, etc for computers of this size? Not mentioning installing new computers.

I am just thinking how a fully automated solution will look like. You would need an entire infrastructure to keep hundreds of thousands of computers in a single room (including the weird requirements for power, ventilation, cabling, etc). And some sort of robotic arms to install new computers or to replace defective ones. And a robotic crane to move around racks, etc. And everything automatic, fully driven by software...