@davecheney 

It never occurred to me to use martini.ResponseWriter
Not sure why....

If I had to guess I'd say that the martini examples, std lib familiarity, and promise of "ful compatibility with the http.HandlerFunc interface" are to blame.

Coming from someone fairly new to the framework, I personally feel that if I try to use a http.ResponseWriter it should behave how one would outside of martini.  Just my two cents