Vicki Davis' post on selling advertisements on her blog made me hopping mad. She describes three instances where companies have contacted her to purchase advertising, two of which were mind-boggling. One company asked her to change old posts for context ads and another suggested that she change STUDENT work for context ads. I agree with Vicki that those two cases are reprehensible.
The decision to sell ads on a blog is up to the individual blogger, but should be clearly disclosed. Furthermore, I don't believe there is ever a case where a previous post (or the work of another person) should be changed to include advertising.
Here is Vicki's post and the Blogger's Code of Ethics which she references. These are important ideas to think about.