They've even released a few nifty UI controls (widgets) that seem pretty feature rich. These include a Calendar, Slider and a TreeView control. The slider control seems very solid and I like the "task" mode of the TreeView control (it allows you to place checkmarks next to the tree items—which would be useful in removing nodes from a tree.)
I'd also recommend checking out the new Yahoo! User Interface Blog, which is designed to talk about changes and additions to the UI Library. They've also published a Design Pattern Library which talks about common design problems and provides some solutions. They Design Pattern Library has an interesting entry on "Auto Complete", but they have not yet posted any actual code.
This is definitely a project to keep your eyes on.
NOTE:One annoying thing about the UI Blog is it doesn't appear to have a straight RSS feed. There are syndication feeds, but I couldn't find a link to a straight RSS feed. Seems like a major oversite.