Check out this video preview of Yahoo Messenger for Windows Vista.Uses lots of whizzy WPF and Vista tricks like transparency, scaling and sidebar gadgets. From an interaction design perspective my favourite bit is the voice-level indicator. Gives users good feedback and helps them feel in control. Coincidentally, I did some conceptual design work for some similar functionality for a company here in Australia almost exactly a year ago. Some of the stuff we came up with was very similar. Didn't think of tabbed conversations though. D'oh.
Where they say: The team also decided that "a new coat of paint wasn't really going to cut it if we want to wow users, and so we decided to work closely with Microsoft and build Yahoo Messenger for Vista,". This is an important point - user experience design is about so much more than eye-candy.