Nick Carr led me to the Washington Post which referred to this intriguing Websense report. Excerpt:

It is observed that at this stage bots (or bot-infected machines) are trying to sign up as many accounts as possible with Gmail mail services. One of the main concerns here is attacking CAPTCHA. Unfortunately, spammers seem to have success with it. The bot is signing up an account feeding all the prerequisites or input data that goes into the signup page and successfully creating a mail account.

Considering the normal / routine process involved in signing up a web mail account (Gmail), CAPTCHA authentication is a must for a successful signup. Since a bot is creating an account successfully, it is obvious that CAPTCHA is broken…