Advertising Doesn't Work; PR Does
Advertisers are well aware of the fact that advertisements do not work. Joel linked to a book on PR by a PR firm.
If you want to see the more dastardly side of PR, you should read "Toxic Sludge Is Good For You" by the good folks at PR Watch: http://www.prwatch.org
It details how PR advertising works and techniques. They know that "consumers" (aka, you and me) don't trust advertising any more. So they try to create fake grass roots campains (this is known as "astro-turfing") and plant news stories which are run completely uncredited but mostly unedited.
In short, after you read this book, you will be amazed at the amount of "news" which is planted directly from advertisers.
It's worth checking out from your library or picking up at your local bookstore. You owe it to yourself to read this book. There's more information and excerpts available here: http://www.prwatch.org/books/tsigfy.html