Without the backing of a major "messenger" service, the appeal to buy Mac may be greatly affected. As good as iChat is, it's worthless without a chat service.
Yahoo has never been very Mac friendly...MS has been luke warm with AOL being the only Mac sanctioned service.
However with MS possibly buying a stake in AOL, does Apple have a backup plan?
http://news.yahoo.com/s/ap/20051107/.../aol_microsoft