As a Kindle with Special Offers user, I don't mind the ads an special offers for the screensaver or the small, non-blinking, banner at the bottom of the Kindle's contents screens.
However, I would absolutely abhor any kind of advertising in the book. Advertising in a book takes you out of the story.
As MrsJoseph said, I really don't want to see those long excerpts from other books in the file either. I've also ran into the 40% of the file is an excerpt situation. It totally mesed up the reading experience as I thought there was a lot more of the story yet to read.