With over 575 million registered iTunes users, it's almost surprising that Apple hasn't already given those users a way to pay for 3rd party online purchases through an iTunes account. The company dabbled in making a payment feature for Passbook, but never saw those intentions through. And we know iTunes has the capability—right now, Apple retail stores allow customers to pay for products with their iTunes accounts.
Of course, this is all speculation at the moment. Apple wouldn't officially comment to WSJ about the rumor, and those who spoke with WSJ did so anonymously. But the mobile payment segment is heating up, and if Apple gets in the game, it could speed us up to the day where we replace our wallets with smartphones. [WSJ]