New confirmed reports from Bing Forums shows that MSNBot is hiding itself under the UserAgent of Mozilla/4.0. How does this person know that it is MSNBot? Two reason, it is "crawler-acting-like" and also because it is in the same range of IP adresses as MSNBot.
Brett Yount from the Microsoft Bing team confirmed the issue saying:
We've received word from a few other sources concerning this. Our team is reviewing it and will let me know as soon as they have an update. Please be assured that it is not our intention to misrepresent the bot or cause undo problems for webmasters.
In addition to this confirmed report, we have a webmaster in WebmasterWorld complaining that MSNBot fails the best business practices of reverse DNS lookup validating. In November 2006, Microsoft added a method to validate MSNBot is indeed MSNBot by allowing reverse DNS checking of the IP. That began to fail in December 2007, which was confirmed shortly after. Then again, in March 2008 the reverse DNS complaints came back. This is a brand new report after not hearing complaints for a while now. There has not been confirmation of this from Microsoft.