I am getting a 500 : Internal error, the payload of the POST is definately right, one thing I noticed is the Content-Type is not "sticking" using b.addheaders = [('Content-Type',"text-x-gwt-rpc")] -- and I'm not sure why.
Does anyone have quick/dirty code used to access a GWT RPC (i.e. emulate the GWT client) in Python using mechanize.  Browser?
The answer to the question is GWT requires the content-type to be x-gwt-rpc; rather than                   'application/x-www-form-urlencoded'; which is hard coded in _http.py in the mechanize library.
Quick and dirty hack is to change this to text/x-gwt-rpc; charset=UTF-8 -- I'm sure this could be done better by subclassing AbstractHTTPHandler; maybe someone else could contribute a quick way to do that.
Calling a GWT RPC function from Python-mechanize