It's going to happen. It's not if, but when. I'm talking about the day when you'll find ads throughout ebooks. This Fast Company article provides a nice overview, including some info on a few related patents recently filed by Amazon.
If you're a purist and who's totally offended by the idea of ads in a book, get over it. My guess is you'll have the option to pay more and still get ad-free ebooks. Tightwads like me, on the other hand, will gladly deal with the minor inconvenience...and we'll save a few bucks along the way. Heck, I miss the ads in the Kindle magazines I subscribe to, so what does that say?
The world of content is rapidly changing, including how it's monetized. More options are a good thing, no matter how bad this idea sounds to you today. I'm just curious to know more about what Amazon plans to do with the money they'll earn from all these in-book ads. Will they pass any of the income along to publishers and authors?