The Bing Blog announced that Microsoft is retiring MSNBot in place of BingBot on October 1, 2010.
So instead of MSNBot showing up in your log files, you should see BingBot in your log files. It should look something like:
Mozilla/5.0 (compatible; bingbot/2.0 +http://www.bing.com/bingbot.htm)
Microsoft said that BingBot will continue to respect and honor the msnbot directives in the robots.txt file, so you do not need to update your robots.txt files yet. But if you are doing more advanced checking, you may need to update your personal scripts. It is also to note that if Microsoft "detect separate sets of directives for bingbot and for any of the older versions of Microsoft search bots (such as msnbot) or a set of directives for all crawlers, the directives for bingbot will take precedence."
WebmasterWorld's Bing moderator said, "So anyone herding msnbot using .htaccess or other means will need to start adjusting your software accordingly."
Forum discussion at WebmasterWorld.