From ZDNet's Zero Day security blog:

"Breaking Gmail, Yahoo and Hotmail’s CAPTCHAs, has been an urban legend for over two years now, with do-it-yourself CAPTCHA breaking services,
and proprietary underground tools assisting spammers, phishers and malware authors into registering hundreds of thousands of bogus accounts for spamming and fraudulent purposes.

Monitoring the [underground service mentioned] service for over a month now, revealed that during the period its “inventory of automatically registered email accounts” was emptying itself,
then restoring to its current position – in the thousands, with 1 to 2 new accounts registered per second.
...."

Link:
http://blogs.zdnet.com/security/?p=1418