dstiles - 7:17 pm on Aug 31, 2011 (gmt 0)
Neither SE publishes their bot IP ranges. They do have valid rDNS for their bots, though. Except for those that don't, which we've asked bingdude to follow up for bing/msnbot. Googlebot seems not to run on "bad" IPs but they do run a load of other junk on both bot and non-bot IPs.
I've run DNS lookups looking for both googlebot and bing/msnbot (and others). Bing/msnbot is terribly disorganised, returning at least 140 "valid" rDNS ranges, many only a handful of IPs within a /24. Apart from that there are several ranges that bing/msnbot runs valid UAs on but with no rDNS; these get banned here.
Googlebot seems to concentrate all its IPs within a /23 and a /21-ish range, which makes it easier to keep track of true bots but I still reject those that have a non-googlebot UA.