The ABC Network in the US was the first to offer programming via iTunes (including Lost and Desperate Housewives). That gave viewers a choice of paying for an ad-free version as opposed to programming VCRs or DVRs to achieve the same thing. It also allowed viewers who had missed past episodes to catch up. So there was a mixture of potential convenience and freedom from ads that may have driven purchases.
Now, ABC is going to offer streaming of its shows with ads but for frees. It will literally broadcast them over the Internet. As usual, what the quality of these is will be an issue but it will certainly make the iTunes versus ABC download choice clear -- ads or no ads. And the price to be free of ads -- $1.99. In the case of daily shows, such as The Daily Show with John Stewart it is less, aout 62 cents an episode (saving 9 minutes of ads in a 30 minute slot).
Here is an interesting Slate article about whether ads versus pay will dominate. It reaches no conclusions but states lots of fears that consumers will be worse off. I am not so sure and am working on this problem. I am haven't cracked it yet but one thing is for sure: don't expect to be paying for shows that are low quality. For those, the only way people will watch them is if they are free and the only way broadcasters might make money is by peppering them with ads.
Expect more updates on that research in the future.