Leah has posted over at the Windows Live Messenger blog some screenshots and a feature list of the semi-public beta build 566. Yes its still not fully public, however signup at ideas.live.com is reportedly taking less than a day to complete.
Unfortunately given the reaction from people on my contact list, it doesn't seem like many users are actually wanting to upgrade to the new build from 365. Many were expecting several new features to keep them occupied during their hours at work and instead have seen the team fiddling with the UI, a UI that has now created a love/hate split. If you take the time to read the comments on Leah's post, you'll see exactly what I mean. I'm saving judgment for the time being as I know the team isn't finished yet.
Leah also confirmed that the build would be available today, no surprise to those of you who read this post and know how many days February has. One point to note is that although I was wrong about M5 appearing this month, it will definitely be out later this week :-)