This code is copy and pasted verbatim from the official reCAPTCHA project: http://code.google.com/p/recaptcha/source/browse/trunk#trunk/recaptcha-plugins/python/
Plus some django widget code copy and pasted from here: http://lobstertech.com/2008/aug/27/integrating_django_recaptcha/ (which also has useful examples of how to use it in your views)
Full credit to the authors!