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> However, an added option which leaves the current behavior as the default shouldn't hurt.
My opinion too. urllib is sometimes a client, sometimes a library used to build clients, which need a knob to implement their own decisions or possibly ask the user for confirmation.
A new argument being a new feature, this patch must target 3.3.
Some comments on the patch:
+ # NOTE: Setting redirect_post_data to True *can* introduce security
+ # issues and is not recommended unless you are sure of where the 
+ # POST data is being redirected!
I would tone down this note, for example:
 # setting redirect_post_data to True can introduce security
 # issues, use with caution
+ redirect_post_data = False
Is an attribute okay or should methods (__init__, maybe methods that do the requests too) grow a new parameter?