Alex @ mp3tunes.com
It's not documented yet because it's proven to be one of the more difficult aspects of the service to get just right.
Our stance is that we will not intentionally break third party sync clients, but may still change the syncing aspects of the api (i.e. unintentionally breaking third party sync clients)
We're currently working on a tcp status and control interface for LS3 to enable third party app integration btw, so you might want to check that out once it's completed. Some of the docs for it are already public (work in progress) here: http://wiki.mp3tunes.com/index.php/L..._XML_Interface
Right now this interface doesn't offer very fine grain control options, but this should improve over time.