Google added "click to buy" links to select videos on YouTube this morning, offering links for purchasing Amazon MP3s or iTunes tracks from music videos, or video games from their trailer clips. It's not the first, but it's one of the service's more direct attempts to justify YouTube's $1.65 billion purchase price and monetise the service. Which begs the question: What's the tipping point for users of the easy-to-watch, easy-to-embed service? Would you be willing to watch pre-play, post-play, or mid-clip advertisements, a la Hulu? Or would anything more than a few display ads along the side move you somewhere else? Let's hear your take on the boundaries of an appealing video service in the comments.