Some other famous sites that are doing this since the mid 2000s - http://decaptcher.com
They integrate with just about every famous spam tool.
UPDATE: Automated Gmail, Yahoo, Hotmail account creation, etc. They create accounts in bulk and then spam through the gmail, yahoo, and hotmail accounts by tools that are being sold.
If you're going to forum profile spam, you'll go broke paying for captchas.
But for other stuff that I mentioned in my previous comment, it does make sense for them economically and they do use it.
Using something like Celerity or Watir for Ruby, combined with this captcha service, you can essentially automate the entire process.
Building a bot to do something like that really teaches you a ton about how you would prevent such activity on your own systems.
Care to share some of what you've learned? Or is this an exercise left to the reader?
1) if you're using a popular CMS platform, ELIMINATE ALL FOOTPRINTS. Change all url strings from the default, remove standard text/descriptions on signup and comment forms. Kill anything that can be scraped against the rest of the installs to hide your own sites from the scrapers the spammers use to find you.
2) Tap into distributed spam prevention systems. Akismet is probably the most popular example. Your single site will most likely miss the indicators of a spammer, but a system like Akismet can see the 10000 links all pointing to the same url in one hour and lock things down for you.
I've legitimately thought about doing a talk at a Wordcamp one day called "How I Spam You" that just walks people through how to spam wordpress, so they can then go protect their sites.