The gist of the GDC presentation is this: many games that take advantage of threading do so by dividing the different "parts" of the game among different threads: one thread for the AI, one thread for graphics; one thread for combat resolution, etc... while this is more efficient than only using one thread/processor, the various threads would still have a lot of down time as they waited for processes in other threads to resolve. A lot of down time. Not only that, but the game could only take advantage of as many threads as the game had been programmed to use.
With Civ V, they moved away from dedicated threads, and broke every thing down to their most basic jobs (or messages): "do this combat calculation"; "animate this graphic"; "determine if this AI will accept this trade"; "what does this civ's strategic AI want to accomplish this turn"; etc...
The have a master message handler that divvies up the messages to the next available thread. There is no memory allocation; there is minimal down time. And, more importantly, it allows the game to (theoretically) take advantage of as many threads as there are available.
A few bits were a little over my head, but that was what I took away from the presentation.
The game engine they were using would be more than 6 months old by now, but they were running the full graphics (all unit & terrain animations, all the water effects, etc...): at one point in the demo he went into the world builder and added ~5000 warriors (maybe 300+ units) to the map. The game didn't blink - not while he was adding the units, not once they were all being displayed. He compared it to civ rev in which their max was ~100 units (or 300 figures).
