premshree has an announcement on his blog and Bill Scott has one on his.
Now, why do I think this is cool? Well, look at what we have. There's a bunch of widgets and utilities. Animation, Connection Manager, DOM, Drag and Drop and Event are the utilities, which to me are more important. Just a brief look at these makes it apparent that you can actually build a Yahoo!/Google Maps product using these five (well, you prolly need only 4, but connect can give you cool stuff). If you've every tried doing drag and drop on your own in a cross browser fashion, you'll be impressed and extremely happy with this code.
The widgets include a calendar, a slider and a treeview. I don't like tree views for web development, but calendars are cool. Let's see someone do a flickr daily pictures calendar.
So, go forth use and multiply this code. It can only be good. Web 2.0 for everyone.
Update: Somebody's done the flickr calendar I mentioned above.