I have the IP ranges below enabled for msnbot. I ONLY allow the official bingbot UA on these ranges so if (eg) a beta bot, browser, mediabot etc hits on the ranges they are rejected.
These ranges were checked last summer using Microsoft's own DNS servers to download the rDNS values but are probably not complete (I didn't check all MS IP ranges - takes far too long). Not all IPs in the range carry bot identification but enough to make it efficient; I currently have about 200 disabled sub-ranges which were too many to manage.
If you run linux or a system that can run dig then it's easy enough to hunt the IPs down given a starting point or three.