Actually, I figured it out this morning. Here is what I found:
I was using the Client API Callback pdf as a guide, which demonstrates triggering the process from a button as follows:
mybutton.attributes.add("onclick", GetCallbackEventReference(Page, "'test'", "successFunc", "this", "errorFunc")
This would cause the error every time. I made the following change and everything worked fine:
mybutton.attributes.add("onclick", GetCallbackEventReference(Me, "'test'", "successFunc", "this", "errorFunc")
Notice the only change is in the 1st param of the GetCallbackEventReference method. Since I am working at the module level, Me is the appropriate reference, not the page that the module exists on. I knew it had to be something obvious and minor.
All works great now! Awsome work on the ClientAPI and thank you. This API is miles ahead of anything else out there and your documentation is a real jewel. I have learned more about AJAX, Client-side scripting and RPC in two days of reading your articles than I have with everything else I have found over the past 6 months or more.