After writing a report which shows inventory transaction cost as of a specific date we had a small issue where very occasionally certain transactions were showing double cost.  After some poking around I found this post:

http://fedotenko.info/?page_id=31

The developer goes into depth about the procedure conceptually, technically, and differences between AX 4.0 and 2009 costing.  It is the very best explanation of the inventory costing feature in AX and saves many many hours of reading and interpreting code to understand.