Writing in Fortune, David Kirkpatrick says:

Technologies often have unintended or unexpected impacts. I know, for example, that I have watched considerably more TV in the years since I got a Tivo. While I may be fast-forwarding through ads, I see far more ads than before, when I almost never could coordinate my schedule with shows I would have liked to watch.

So it may be that the value of ads (and their price) will actually increase as a result of Tivo, on the theory that the valuable audience to advertisers consists of those who are interested enough to slow down the replay, and that the fast-forwarders would not have bought anything anyway.

But note that this outcome, which would be win-win-win for advertisers, broadcasters, and viewers, is dependent on the property rights of the broadcasters, which gives them the clout to cut a deal with Tivo whereby commercials can be fast-forwarded but not eliminated entirely. (Tivo originally obliterated the commercials.)