The May 2009 "Oslo" CTP is available on the Oslo Developer Center! Shawn Wildermuth has done a nice job of summarizing the changes to the M language since the January CTP. Also check out Jeff Pinkston's (aka "Pinky") blog for some existing posts on the changes and more posts on the way.
In addition to updates to M and the Repository, this is the first time Quadrant is being released to the public. I worked on that test team before coming to the M test team so it's great to see those bits in the wild!
Thanks to everyone on the team for their hard work. We eagerly await your comments and questions in the forums.
[UPDATE] Here is an even better link for "what's new" in the SDK.