AFAIK MS didn't publish that URL officially yet, it only started showing content just now. A few things on there probably still need fixing.
There is also http://www.messengerpreview.com/
but at the moment of posting it shows an old CES video for the Arch keyboard, which seems unconnected from Messenger. Maybe they're still holding some stuff back until they made the official announcement.
Supposedly there will also be beta invites going out today/tomorrow (depending on your timezone). And the public beta is rumored in
3 weeks from now
or more like 6 weeks if you believe others
. Either May 18th or June 7th.
Source: Windows Live Wave 4: Things Are Coming Tomorrow (mymicrosoftlife.com)