Visa and DeviceFidelity have announced [pdf link] their contactless iPhone payment system, In2Pay, consisting of a special case for the Apple smartphone and a microSD card that contains a Near Field Communications (NFC) chip. The case itself has a microSD slot that's interfaced with the iPhone's dock connector; when paired with the In2Pay Visa app from the App Store, you can tap your iPhone to make payments in participating retailers.
Video demo after the cut
Since the NFC technology is embedded into the microSD card rather than the iPhone case or the iPhone itself, there's a degree of cross-platform compatibility. Assuming Visa offer a suitable app for your smartphone - and that said-smartphone has a microSD slot - you should be able to continue to use the modified card even if you decide to dump your iPhone.
Of course, Apple aren't letting the NFC grass grow underneath them, and the company has already filed patent applications for various NFC-based ticket and mobile payment solutions that would be integrated into iPhones and other mobile devices. There has been no sign of NFC hardware in the leaked iPhone HD prototypes we've seen to-date, but it's possible that Apple could announce the functionality in their fourth-generation handset at WWDC 2010.