Yahoo just released their Yahoo! Messenger for Vista.  I haven't yet installed it but I do know that's its based on WPF.  Check out some of the blog postings about it, below.

http://eric.burke.name/dotnetmania/2007/12/06/0.05.32

http://blogs.msdn.com/tims/archive/2007/12/06/great-wpf-applications-16-yahoo-messenger-for-windows-vista.aspx

http://blogs.zdnet.com/Stewart/?p=661

http://www.ymessengerblog.com/blog/2007/12/05/yahoo-messenger-for-vista-preview-release-available/