Joining Windows Live OneCare, Microsoft Money, and Encarta, MSN Web Messenger is slated to retire on June 30, 2009.  Its replacement, Windows Live Web Messenger, accessible through Hotmail, has been available for a couple of months now, and after June 30 will be the only way to access Messenger through the web.

According to the Windows Live Hotmail team blog:

With new web-based instant messaging (IM) now available from Windows Live Hotmail worldwide, we are preparing to retire MSN Web Messenger. The old MSN Web Messenger experience will end on June 30, 2009

via Ars Technica