I have to wonder if there have been some recent changes which are causing all kinds of issues. Just a few days ago, Ryan Tomayko twittered about it completely ignoring Github's robots.txt - http://twitter.com/rtomayko/statuses/7685967826
Our Mysql boxes were hammered thanks to this bug. Pages that weren't supposed to be crawled because they're resource intensive were all getting hit. Very frustrating, we ended up blocking all msnbot traffic in Nginx.