> The only way I can think of doing this is to just rerender the entire 
> frame multiple times - which would obviously not work in realtime. So

That's one way of doing it, and it works ok if your scene is simple enough, 
or if you only want to motion blur certain parts of it (eg a fast moving 
missile).

> I'm not sure what clever trick they've found to make it fast enough...

Another way is to render the scene to a texture, and at the same time render 
the velocity information of each pixel to another texture.  Then a final bit 
of code (running on the GPU naturally) creates the final image each frame 
using that data.  This method maybe slows down rendering by 5-10% on a 
complex scene, but allows full screen motion blur, all you need to do is 
make sure you add some velocity data to each mesh you render.

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