Bingdude - thanks for the response.
Bad rDNS - 22.214.171.124 - 126.96.36.199 is a good place to begin - there may be others but those are the biggest hitters. Some of those IPs have been actively bot'ing since October 2010 right up to date. As far as I have checked, none in that range carry the bot rDNS: they are generally (all?) msft.msnhst.etc. I did a DNS scan and grepped about a month ago but found no crawl references.