Check it out: a beta version of Yahoo! Messenger for Vista is now out. Download it here and read about all the new features and see a video here. This is 100% WPF -- hats off to Eric Burke and company who have done a knock out job with the application. He says on his blog that he'll be posting lots of learnings about WPF; looking forward to that.
My favorite feature? Check out the emoticons, for example, how they composite on top of the application. Pretty sweet. Give it a shot and provide feedback to the team on what you like and don't like.