I've begun documenting the MythTV protocol in detail in a wiki. It's far from complete, and is probably more incomplete than some of the other protocol references on the internet. However, I do have about 70% of the protocol figured out and documented in comments in the library. If you know a little bit of Python, you can probably figure out everything you need from the code.
Note that this is not the final protocol API, only the initial structure. I am planning to write some object oriented wrappers around the protocol after I've got all of the commands implemented.
If you've got questions or comments, send me an email: synack <at> csh.rit.edu.