By the middle of next year, users of both are expected to be able to exchange instant messages, see if their contacts are online, share emoticons, add friends from either service and make PC-to-PC voice calls...
...The alliance could turn up the heat on people using Yahoo IM. Microsoft's network is the most popular object of IM worms, as 62 percent of attacks in the third quarter this year were aimed at MSN Messenger or Windows Messenger, IMlogic said. In the same period, only 7 percent of IM worms went after Yahoo Messenger.
Interoperability comes at a cost in the form of security risks.
Link: ZDNet: Microsoft-Yahoo alliance is food for IM worms