Yah I asked him for the details in the comments and he said that WP Engine runs the following on their servers:
NGINX (reverse proxy)
Apache (webserver)
APC (PHP Accelerator)
CDN baked into the architecture
Insanely powerful hardware
NGINX (reverse proxy)
Apache (webserver)
APC (PHP Accelerator)
CDN baked into the architecture
Insanely powerful hardware
It's certainly not hard to figure out what they're running of course. http://builtwith.com/wpengine.com and the "Net" tab of the Firebug plugin tell most of the story.