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.