DeviceFidelity just got a nice seal of approval from Visa this week. They announced that Visa has certified their microSD card contactless payment solution that works like the current credit cards that let you tap and pay. I am not sure if they are based on the same standard but hopefully they are interoperable. The main downside to such an application would be that it would take away your microSD storage or limit it to what is available on these cards but hopefully more companies integrate solutions like this in the future.
Currently the device is certified to work only on the BlackBerry Bold 9650 along with the Samsung Galaxy S Vibrant, and the iPhone 4, 3GS and 3G. Not sure why it would be limited to the 9650 but I doubt it will stay that way.
I don’t think that such a MicroSD based solution will take off. I think it is up to device manufacturers to build this tech inside the device and DeviceFidelity made some headway on that front with this certification.