Near field communications and online BC banks/wallets will make this simple. In due time something like Google Wallet could work, but instead of having to use a credit card to pay, you can put real money into your Google wallet and pay from the stored credit. Tap a phone on the payment device, confirm the transaction and you're done. If retailers can be convinced that they can get the perks of mobile payment and NFC without having to pay the merchant fees associated with credit cards, it'll be a strong arguing point.
NFC is years away from being prominent though, and is only beginning to be tested. For now, online wallets work well enough for mobile payments and I think it'll take a wide range of compatible NFC devices for the best transition into a portable option.