The Bing Search Blog announced that on October 1st, MSNBot will be renamed to BingBot.
Bing has been testing a beta bot since November of last year. On October 1st, the beta label will be removed from the Bot name and it will be named BingBot.
You should see the following useragent in your log files:
Mozilla/5.0 (compatible; bingbot/2.0 +http://www.bing.com/bingbot.htm)
Note, if you already have directives for MSNBot in your robots.txt file, BingBot will obey those directives. It is important to note that if you have two different directives, one for MSNBot and one for BingBot and they say two different things, Microsoft will listen to BingBot over MSNBot.
For more information, see the Bing Search Blog.