You are usually not directly connected with your contacts but connected trough the msn switchboard, meaning, you get the messages from the msn servers, and you'll have to send a file or do something else that will create a direct connection, and then it would still be hard to find out the correct ip of your contact.
But IIRC there was a 'plugin' that did this, maybe you or someone else can get it working with a 'plugin-wrapper'.
EDIT: It was a normal script, but it works with Xsniff, which doesn't work that well for some people.
http://shoutbox.menthix.net/showthread.php?tid=64308