I was speaking of individual machines, not of the whole server farm. The total load served by Amazon or TM is quite impressive, however they use a lot of machines to do it. From what I know (more about TM than Amazon) though, the query rate handled by a single machine is not huge.

Yes, we really do use cooperative multi-blah systems at Yahoo, at least for some applications. Some of these more resemble a threaded programming model, some are more like event-based state machine models.

The most common place that I can think of where you'd like to use a design like this is when you have a service-oriented architecture and the server that originally receives a user request does little more than analyze the request and then make a number of sub-requests to other services. An event-driven state machine is generally the most efficient and scalable way to multiplex really large numbers of simultaneous connections. With the Apache pre-forked process model, each child is tied up for the whole time that you are waiting for the subrequests to return. There's a fairly low limit on how many active processes you can have before your machine ends up spending more time context switching than doing useful work.