I ran rDNS checks in bulk a couple of weeks ago and came up with the following. There are a few blank rDNS entries of one or two IPs within some (most?) ranges which I included to reduce the number of ranges I had to use in the software.
NOTE: These are only for an rDNS that includes msnbot.
I'm gradually building up a new db of good (or at least acceptable) bot IPs 'cause I can't rDNS them on the fly. So far google seems most reasonable in allocation: six C's in two clumps. Yandex is definitely the worst - about 150 IPs scattered two or three to each C range; and they still send round oddly titled rDNS with their standard bot.
Update on the 207 ranges. Not sure if this was me being sloppy or if some of them are new.
Note: some IP sets are msnbot-(ip).search.msn.com and others are msnbot-(ip).msn.com. No idea why but it seems as though the ones I missed before are of the latter persuasion (which may be why I missed them).