Does this represent an opportunity for book publishers and authors? Absolutely. Hear me out, even if you're one of those purists who insists on books existing as they always have, without ads...
Let's start with the fact that you could always sell two versions of your products: one with ads and one without. The version without ads is priced higher than the one with the ads. Test, measure, rinse, repeat. Why wouldn't you want the opportunity to study real world data from your customers to see whether ads have an impact on product acceptance and sales? The results might just surprise you.
Or, how about all those free samples you have floating around? I'm talking about the excerpts you distribute in the hopes that you'll convert some number of browsers into buyers. Your conversion rate is something less than 100%, so why not find other ways to monetize that experience?
This is yet another one of the Kindle platform's shortcomings. Amazon never built a model content owners could leverage to drive some additional revenue. They didn't offer it with books but I'm even more amazed that they never figured it out for newspapers and magazines, especially since we're all quite used to seeing ads throughout those products.
In case you haven't noticed, most customers feel ebook prices should be lower than print prices. That pricing pressure alone should cause every author and publisher to experiment with services like iAd. What have you got to lose?
P.S. -- If Apple is smart they'll build iAd into the iBooks app. You'll tell them whether or not you want iAd service included every time you provide them with ePub files for your next book. Hopefully they'll also let publishers try out that two-pronged approach where the book is available both with and without ads, at two different prices.