When a search engine spider, bot, crawler visits your site, they should all be capable of passing what is known as a reverse DNS test. I have details on how this works over at this article.
A WebmasterWorld thread reports that Google's Webmaster Tools verification bot is not always passing the test. A user said that he first noticed this when he had to keep in reverifying his sites with Google Webmaster Tools. Sometimes the verification worked and sometimes it did not. So he checked his log files to see that some of the IP addresses being sent by Google were not passing his reverse DNS test, thus he was blocking Google for verifying his site.
I have no exact proof of this actually happening but we do have a "me too" post supporting this webmaster's findings.
Forum discussion at WebmasterWorld.
