According to a message posted on February 9th at the MSN Messenger download site, all users on 6.1 and 6.2 are being forced to upgrade to the latest build or be 'blocked' from the service.
The short announcement simply states "This upgrade includes the most robust security measures currently available for MSN Messenger. You will only be able to log onto MSN Messenger if you have upgraded your MSN Messenger software.". A link to the security bulletin provides more information, an excerpt from the page reads:
Microsoft is aware of exploit code available on the Internet that targets an issue addressed this week by Microsoft Security Bulletin MS05-009. Microsoft is not currently aware of any active attack utilizing this code or any customer impact. We will continue to actively monitor the situation and provide updated customer information and guidance as necessary.
Windows Messenger 4.7 is also affected and there is an update available for users running v220.127.116.119 (update) on XP Windows SP1 and v18.104.22.16800 (update) on SP2. MSN Messenger 7.0 BETA users are unaffected.