A WebmasterWorld and Search Engine Watch Forums threads are both reporting issues with Yahoo! Slurp (Yahoo!'s Crawler) indexing pages they should not be, and in quantities that may be harmful.
It appears that only specific bots are not obeying the robots.txt file and indexing pages are rates that can potentially cause server issues.
The specific IP addresses appear to be in the 74.6.x block. They do reverse DNS to inktomi, which is correct.
Forum discussion at WebmasterWorld and Search Engine Watch Forums.