At first glance, this seems like another one of Microsoft's underhanded marketing tactics. Closing MSN to third party IMs would force me (along with many others) to install MSN whether I want to or not (I happen not to). As a big fan of XML, webservices, RSS, etc I just don't have the patience for these “My proprietary IM is better than yours” wars. But before I go off on a rant about MS shafting customers just to improve market share - does anyone have any additional information on why Microsoft is doing this? What are the “security concerns” mentioned in the article?