Thanks for the response.
I've never used Firebug before, but I installed it and took a look around. Just to clarify: I'm assuming that ZmZimletContext._zmObjectTransformers is a function somewhere in the Zimbra code, and you're suggesting that I use firebug to break the execution of this function to see what it's doing; see if it's passing back bad data to the Zimlet I wrote, right?