There is buzz that Microsoft and Yahoo will announce tomorrow that their instant messaging clients will beome interoperable. The catalyst seems to be the surge in Skype and Google Talk usage. Together MSN and Yahoo account for 44% of total IM usage today (not counting Skype and Google), and reach about 33.5 million monthly unique users.
UPDATE: This rumor was confirmed this morning via a Yahoo press release. The companies announced that interoperability for their 275 million combined customers will occur around Q2 2006. Hey, how about NOW? :-)