Looks like MS will be adding some new IP addresses to support Windows Live Messenger PIC & Federation. In case you missed the original posting these changes go into effect Friday May 9, 2009 so you have some time. If your firewall restricts access on a port by port basis then you will want to add these to your access list before the deadline.
New IP Addresses
Check out article 897567 for a complete listing of IP addresses and host names.