Strangley I'd have expected a beta version of Windows Live Mesenger 2010 to have appeared around the end of 2009, so that they could get as much fixed as possible for an early 2010 release, but as it goes it appears even the Beta hasn't been released yet.
However there are screenshots of an apparent upcoming Beta that have been leaked online. These can be found on this article.
http://www.msgstuff.com/news/post/611-windows-live-messenger-2010-leaked-screenshots-reveal-new-features/Interestingly MSN appears to have decided to listen to users for once. Apparently one of the most demanded feature for many years has been tabbed conversations. Now for those who use Messenger Plus, you'll know this is already available (I beleive it's available in Messenger Plus itself, although I could be wrong and it could be a downloable extra script). However its a popular requested feature and there are many out there who will refuse to instal messenger plus.
The new version will also include social network intergration. A rumour for the last version, 2009, also claimed to include upcoming multi video conversations, which proved wrong. This however might have simply been something MSN where developing, and it wasn't ready in time, although there is currently no word about this feature. We will just have to wait and see.