I imagine this will happen via Payment Service Providers first. Merchants that don't want to manage their own payment code outsource it to some third party comp

I imagine this will happen via Payment Service Providers first. Merchants that don't want to manage their own payment code outsource it to some third party company. Those PSPs compete and one dimension they compete on is breadth of payment methods. Who wants to be the only one who doesn't offer BTC? Once PSPs have integrated bitcoins, many merchants will accept them.