68.180.211.nnn is not a yahoo bot range - at least, not slurp. As with most larger engines, a lot of their IP range has nothing directly to do with crawling.
If you do an rDNS lookup on the actual IP it should show you what it really is for. That particular C-net range comprises betas, shops, video search (which may be some kind of bot but not identified as such) but is mostly answers and mobile. Again, these may be some kind of bot but if so they're not being honest about it.
I get a lot of yahoo accesses with absolutely no headers but rDNS returns of things like ycar1.mobile.ch1.yahoo.com. Since they have no UA or other headers they get blocked.