This came up ages ago, iirc Amazon got some patent for interactive ebook ads. The media and some posters got into a big hizzy, imagining that ebooks would have ads every 3 paragraphs and be sold for free.
Unsurprisingly, it hasn't happened yet. And since the economics of web-based advertising are pretty crappy, I doubt it will.
What's more likely is that the publisher might put an unobtrusive ad or two at the end of an ebook, similar to what they do with mass-market paperbacks.