For a while you could not access MSN Webmaster Live to submit or update sitemaps. This was due in part the merger of Yahoo and MSN last October.
There are still two different control panels for submitting sitemaps to these search engines. MSN Webmaster Live is now renamed to Bing Webmaster. I would suggest that you check to make sure you site’s sitemaps are still active in both the later accounts. I have noticed irregularities in some of my clients accounts with these two firms.