Bing Search announced that on October 1st, MSNBot will be renamed to BingBot. And, instead of the old msnbot 2.0b showing up in your server logs, the updated user agent will be:
Mozilla/5.0 (compatible; bingbot/2.0 +http://www.bing.com/bingbot.htm).
The HTTP header From field will also change:
From: msnbot(at)microsoft.com will become From: bingbot(at)microsoft.com
Finally, that bingbot will still honor robots.txt directives written for msnbot, so no change is required to your robots.txt file(s).