Are you using a script to show this information?
Windows Media Player 11 does not support displaying song information from iTunes by default.
If you're using a Messenger Plus! script, it's possible that this script has not been updated for use with Messenger Plus! 5.
Windows Media Player 11's built in method for showing song information in Windows Live Messenger is working fine for me with the exact same software as you, minus any media scripts in Messenger Plus! 5.
Make sure that any scripts used are disabled and the box to show media information in Windows Live Messenger 2011 Options is enabled.
Picture of Windows Live Messenger 2011 options