A couple of users have pointed out seeing the 6.0 version number at the bottom of the Web Mail page today. That's the only thing that changed, so don't look for the 6.0 features yet. They'll be obvious when we turn them on. Right now that code base is just running in production with all the new features turned off while we perfect them.
In a few weeks we'll be turning on the 6.0 features for a portion of our users to phase it in and get feedback, and then it will be enabled for all users after Christmas.