To reproduce:
- Launch a HTTP server listening on an Inet6 socket on, say, port 5555
- Try to connect using the IPv6 loopback: 
http = httplib.HTTPConnection('[::1]:5555')
http.request('GET', '/foo')
r = http.getresponse()
print r.status
- You get 400 (bad-request) instead of 404

It seems that's because the HTTP request is wrong. Python sends this header:
Host: ::1:5555
but it should be:
Host: [::1]:5555

I'm using python 2.5.2-1ubuntu1 on Intrepid.