Just one minor nit: some of the lines in the code are >75 chars long. This includes the script usage help.

Also I prefer not to have whitespace directly after "def" lines.

The do_POST handling of "invalid challenge" could do something more useful. At the moment it'll raise a strange exception ("NoneType has no attribute split", I believe. Either completely squash the exception or let it be raised (I'd prefer the latter as it would aid debugging if the situation ever arose).