For anyone who hasn’t read it yet, Yves Poissant has written a nice tutorial on photon mapping in A:M.
He talks all about indirect illumination, different rendering algorithms that are used and basically just does a really good job of explaining how photon mapping can be useful in A:M.
It’s a good read check it out 
http://www.ypoart.com/tutorials/Photon-Mapping.htm
-David