I tried Brono's idea.  Very interesting results.  I did the exact same render except I changed the Global Radiosity to Ambient Oclusion and reduced the Advanced Lighting Models to -1.

The render time went from 12 hours to 90 minutes.  (And that was with Pirates of the Carribean 3) playing on the same computer.)

There was however a noticible difference in the lighting quality.  I think that the Global Radiosity had better overall lighting.  But was it worth 10 and a half hours of extra rendering time?  I don't know.

Anyway, here is a side-by-side for you.

The left side is with the Global Radiosity and the right side is the Ambient Oclusion.  Very minor difference.