You can hear what Anderson has to say about free here and here. You can read Malcolm Gladwell's dissent here. And Seth Godin's counter here.
But if you're in a hurry, the synopsis is that more and more companies are making more and more money giving things away for free. And the book is full of examples from Google to Webkins to Radiohead to Ryanair to the UC Berkeley.
But there are no examples of advertising agencies (unless you count Google). And if Anderson is right, sometime in the near future, someone is going to figure out how advertising agencies can adopt free. Maybe it's an agency giving free brand consultation. Or creative. Or strategic planning. They already do this to painful extremes when pitching new business. But maybe there's a way to do it with paying clients.
Here's a noble attempt rising from the primordial soup of the free economy. True, they're a creative team and not an agency. But it's a start.
If this all sounds counterintuitive, read Anderson's book. It may change your mind. And if you're willing to share any ideas on how agencies can use "free" to their advantage, I'd love to hear them. I'll post mine as soon as they come to me.