I don't really know, because I didn't write it myself ;) I think it basically logs in into a service provider site with pycurl, follows the right links, reads captcha's and writes an SMS, which is then sent by the provider itself.
I can give you the direct link to the source code from the same site: http://www.moioli.net/files/MoioSMS2.18-src.zip -- http://mail.python.org/mailman/listinfo/python-list
