Here's an interesting interview with Al Ries, author of "The Fall of Advertising & The Rise of PR." Cincom's Steve Kayser asked him a few good questions about advertising and PR - I liked this summation of things:

Steve: So when it comes down to bottom-line ROI?

Al: No-brainer. The largest advertised brand in America spent $780 million on advertising last year. Do you know the name of the largest advertised brand? It's not McDonald's, Budweiser or Coca-Cola.

The largest advertised brand in America last year, would you believe, was Chevrolet. Now let me ask you a question, what's a Chevrolet? If I told you I would meet you out front in my Chevrolet, would you be able to recognize my car?

What's a Chevrolet? A large, small, cheap, expensive car … or truck. But you already knew that. $780 million and there probably isn't one thing stuck in your mind that you can connect with Chevrolet. What a waste.

His point is the large scale advertising is often a symptom of trouble (think about companies that went into bankruptcy recently), or a sign that a company hasn't got any good brand recognition - and advertising isn't going to solve that problem. A good ad campaign can complement good PR, but it can't salvage bad or non-existant PR. You need something to build on, first.

Read the whole thing - there's more good stuff there.