The company set out to change that earlier in the year, fighting off lawsuits from companies like Viacom. Most recently, YouTube has had a new section introduced in the guise of a sales channel. The “sales channel” will place buttons underneath YouTube videos that are links to purchase something related (we'd assume) to the video in question. The actual sales come from third parties – like Amazon or iTunes – and Google gets a commission for a completed sale.
It's not mentioned if the sales channel will stick specifically to an opt-in basis. After all, many people who upload videos to YouTube don't necessarily want their videos being associated with a product.