According to the bitcoin wiki, the data used comes from calling getwork.
getwork is a JSON-RPC defined at https://en.bitcoin.it/wiki/Getwork as "An RPC method used by a miner to get hashing work to try to solve."
After reviewing the source here: https://github.com/bitcoin/bitcoin/blob/142e604184e3ab6dcbe02cebcbe08e5623182b81/src/main.cpp#L3352 I was unable to determine the code for the GetWork method. I am mostly curious about the source of the data though. Although the bitcoin system is said to be entirely transparent, it seems to skimp on information regarding the data used.
Where/what is the origin of the data that is being worked on by the bitcoin hashing algorithm?