Hi onchain !
Sorry for the late answer (was out of office).
Started POST "/callback" for 192.168.178.39 at 2014-09-11 12:07:05 +0200
{"signature"=>"ICmRRZees1ERGP0zFRpjlzNA60e3b9GYAwScGbpqe7Lnek6ulUwTB3VXkWkH1ZbaPirEf60EKrWGAXE0BqMSETI=", "uri"=>"bitid://192.168.178.29:3000/callback?x=463eb9f064a4173f&u=1", "address"=>"1PmcrixixGKzSyL6NbNcSzjHGLSsgb7dfa"}
Regarding the structure of your request, everything seems ok.
I assume the values are those used with the rails demo and that you had different uri, signature & address for your test with the python demo. That's right ?
May you check the value of the field "message" in the json returned by the server ? It should give us a first hint about the problem.