Microsoft and Yahoo! have announced they are releasing a combined, interoperable network for beta testing, which will allow uses of each network to communicate with users of the other network. This is a great step in creating a IM infrastructure without (or at least with less) borders.

"Windows Live Messenger and Yahoo! Messenger with Voice users in the U.S. and more than 15 international markets can register to participate in the IM interoperability beta by visiting Yahoo! at http://messenger.yahoo.com or Microsoft at http://ideas.live.com."

See the press release here.