No matter how hard Microsoft tries to push developers to be excited about Vista, the overwhelming number of XP installs in the world makes it a losing fight to try and develop exclusively for the newer OS. One company who was an early advocate for Vista development, Yahoo, is taking one of their biggest profile programs off the market.
Yahoo has decided to discontinue the development and release of the Vista-optimized Yahoo Messenger Client, instead returning to a one-size-fits-all client that is compatible with various versions of the OS. Yahoo claims the change is to improve efficiency in development and make the program more consistent, and it's very easy to see why - developing two separate code bases side by side for two different platforms can lead to all sorts of compatibility problems.
Yahoo isn't disabling the Vista client's access to the YIM network, but has encouraged anyone using the Vista client to install the standard version.