GUI design Patt Ern-s http://developer.yahoo.net/ypatterns/index.php and UI library (Java Script, AJAX).
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(old notes regarding Pass Port)
Could be one of the Member Sites and/or one of the Reference Sites.
They could be a "third way", avoiding the AOL/Microsoft "evil of two lessers". They already have a huge namespace. They have an instant messaging client, though with a relatively small user base (but they don't block Jabber, so they get big points for that).
They have lots of generic/horizontal services. I could imagine plenty of Independent Developers coming up with products/services that could be nicely complemented by those features. Even if a develop didn't mind building such services, who would want yet another calendar to look up? Wouldn't it be better if your team collaboration tool generated meetings viewable in Yahoo Calendar (Group Calendaring)? Yahoo could become a real Web Services/Hail Storm-type competitor to Ms Outlook...