Every one of the companies listed here http://www.creditcardprocessing.net/
are already set up to process merchant's credit cards. Their business model would only need a slight tweak to add Bitcoin to their existing services. Some even charge a monthly fee for that service thus, at the onset, can exchange bitcoins with a low (if not zero) fee, offering it to their current customers as a bonus. Moreover, they'll possibly gain merchants that only want a service to exchange bitcoins.
I am aware that we already have fine companies set up to complete this task, but mostly for the sole purpose of exchanging BTC into USD or the other way around. This idea is offered up to get those big named entities on board, thereby increasing awareness for Bitcoin. It would only take one or two for the rest to follow suit. The link I provided only shows a handful of these companies, mostly the main ones. But there's a myriad of them available on the net, scattered across the globe.