There is a preview version of the NEW Yahoo Messenger for Windows Vista available and "testers" are wanted.  If you choose to use it,  you can still use your current version of Yahoo! Messenger (8.1 or 9.0).  istartedsomething had some interesting comments that you might enjoy reading.

...the new Yahoo Messenger is possibly the first real mainstream application to take advantage of Windows Presentation Foundation (WPF) and a number of Vista APIs for which I salute them for adopting new technology.

Comparing this preview release to the original conceptual design by Frog Design shows a number of subtle interface modifications but also a major disappointment.

Changes to the interface include the application windows are now encapsulated inside Windows Aero Glass frames. On one hand, this satisfies Microsoft’s Vista user experience guidelines which suggests all windows use the standard frame, but on the other seem rather quirky because the application has two harshly different styles of transparency.

The major disappointment is the lack of voice, video and sharing capabilities which were widely fanfared and demonstrated at CES.

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