What about a discounted price for a book? Would you be willing to pay less if you had to see the occasional ad?

But if you think about it- it's already that way now in print. Buy a paperback and most have excerpts of other books by the same author (or at least the same publisher). Some even have an order form that you can cut-out and mail-in. Granted, they're usually in the back, after the last chapter.

it's also more agreeable when you can turn the page.
i would imagine the e-reader implementation will hold you hostage until the ad runs its course, like many of the video content publishers do now.
perhaps mid-chapter - just when it's getting juicy.

as long as they dont do something like interest-based ads. imagine reading a book to your little kid and something embarassing turn up. you'd never know what's on the next page.
we already know that companies can keep a two-way link to your ereader because of that big hoo-haa when amazon deleted an already purchased book from people's kindles.