This paper looks interesting from Intel, don't know if POV uses something 
like this already, but the speed-up factor compared to OpenRT looks very 
impressive (3x - 10x).  They claim that their algorithm results in only 1/3 
of the traversal steps being required during the trace, by grouping rays 
with similar paths in the scene together (I haven't studied the exact method 
yet).

ftp://download.intel.com/technology/computing/applications/download/mlrta.pdf

BTW I stumbled across this while looking at real time raytracing for the 
Cell processor.

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