I work with mainframes for a living. Specifically, I work with Linux on IBM zSeries mainframe for a bank. The idea is the provide the software depth of Linux with the reliability of the zSeries hardware.
We get a fair amount of resistance from the Lintel bigots, mostly those who still think of the mainframe in 1980's terms. The current generation of mainframe packs a LOT of horsepower, particularly I/O capacity, in a relatively small box. It connects to the same storage and network as the Lintel servers do, but can one of those do 256 simultaneous DMA transfers? We don't sell the platform as a solution for everything, but we've done the TCO math and we're not that different from an Intel server farm once you factor in the external costs.
I periodically give a class to the Linux admins on the mainframe in general, Linux on z, and the differences between that and Linux on Intel. If you didn't know where to look, it would take you a while to figure out you're not on Intel anymore. Most of the attendees are surprised at what the current boxes are like.
This is not your fathers mainframe.