There *is* a bug, but it should be generating text/x-json (and that should be fixed). Are you sure you're getting text/html?
I definitely received text/html. (web2py 1.98.2, running on rocket 1.2.2). A wide variety of content types seems to be in use, buthttp://www.ietf.org/rfc/rfc4627.txt specifies jsonrpc must have "application/json".
Best regards, Stefaan.