Okay, I'm surprised at myself that I found this, and I don't think anyone's covered it. I was just reading the Analytics Blog... and Wahlaa!

services.google.com/websiteopt ...

via

analytics.blogspot.com/2006/10 ...

Seems like its invitation only, and part of the AdWords family (in conjunction with Analytics)

Here's a screenshot of what it looks like:

www.google.com/analytics/siteo ...

Signed up for an invitation. Hopefully I'll get an invite soon. Good find!