Many thanks to John for this heads up.
T"his message is to inform you that the Microsoft .NET Messenger and Web Messenger service will be unavailable due to system maintenance starting at 8:00PM PDT on October 29th.
Service is expected to be fully restored by 1:00AM on October 30th."
Microsoft have issued no comment on the reason behind the outage. One might speculate that the outage could be to add improvements to the service in preperation for MSN Messenger 7.