This is great news. The first beta came out in August so this has been a long time coming. Check it out here.
Update: Funnily enough, none of the links that seems to be available actually point to the Beta 2. Click on them and you get the old Beta 1 version. I did track down a valid Beta 2 link in their dev site (http://dev.live.com/writer/). You can download the Beta 2 from here until they fix the links everywhere else.