Half of the users get the new ones. They're seeing if these one's are easier than the ReCaptcha ones by checking the fail rate. Join the Tech Team Usergroup, they tell you about these things.
I've noticed that reCaptcha tends to serve up non-text or totally incomprehensible text fairly often, so we're going to test a different captcha system on about half the users (called Solvemedia). This one can mix phrases, animated images, or ads into the captcha instead of the reCaptcha scanned text images - the ones that I've seen so far tend to be a lot easier to understand (the ad-based ones are in clear, plain text).
Some people will continue to see the reCaptcha units, and won't notice any change at all. I'm tracking how many failures each type generates so I can figure out which one actually causes the least number failures.
The Solvemedia tech is based on the same stuff as reCaptcha, so it has reload and audio options too, which most of the other third-party systems don't seem to"
This is the PM that the Tech Team group sent out to all members of the usergroup.