http://blogs.msdn.com/livesearch/archive/2006/11/29/search-robots-in-disguise.aspx

Hé hé, funny ;)
But I'm sure that it exists bots (from Google, Msn, ...) which does not have a usual user-agent and the reverse DNS from the IP doesn't seem to come from official servers.. ;)

Sounds like similar advice to this post:
http://googlewebmastercentral.blogspot.com/2006/09/how-to-verify-googlebot.html