It's header in big-endian hexademical format with additional bytes for SHA256 (see wiki on SHA256 for details).
Can you give a link for this? I've search all over and can't find anywhere that explains how the padding should be done? I'm currently trying to work out how to turn a header in byte array form back into a valid getwork hex string.
You want section 5. Note that the message length is stored in Intel's crazy-endian format.