This was a talk given at GDC 2012, the morning after the SimCity announcement, on the internals of our new simulation engine, called GlassBox. This is what we're using to ship SimCity.
It covers the basic pieces that make up the simulation architecture, the things we considered in putting it together, and explains why we took this approach.
Videos from the talk:
And here's some shaky cam footage from the talk itself:
Dan Moskowitz (who showed off various SimCity examples after the slides) has more here.