This question helps identify the various components of the getwork "data" field, but I'm curious where the extraNonce comes from. My hunch is it's the first four bytes of the second half of the data, because I believe this is still part of the Merkle root (and yet, being in the second half of the data, it won't be part of the midstate). Is this correct? And if so, how does one increment the extraNonce, and what does it mean to do so?
Example getwork data:
Is the extraNonce (in little-endian, I know),