There is a concept that when teaching children you need to be careful in the way you act in front of the kid. Kids learn by example more than they learn by what they are told. This also applies to adults - really does.
A WebmasterWorld thread notes that a person spotted a a Yahoo! Publisher Network terms of service violation on a "big site." He said, hey, if this site is doing it, does that means it is OK?
No, it is not OK.
The specific violation is that Yahoo! doesn't allow you to run other contextual ad programs on the same page as a Yahoo! contextual ad. Google recently removed that clause from their TOS but added a different restriction. So Yahoo! can and would boot you, if they caught you violating this program policy.
So if you see stuff on other sites, be it contextual related, PPC related or SEO related - do not assume it is allowed just because they did it. Heck, BMW spammed they got kicked out of the index. If you would have copied them, you would have been kicked out too, if caught. Now, you are now BMW, and unlike you, they got back in within days, because they are big - but you are not. So be careful.
Forum discussion at WebmasterWorld.