[tlb]

It's unlikely to be faster. I can pay with cash or credit card in much less time than my iPhone takes to get a GPS fix & 3G connection indoors.

Also, credit cards are very reliable. I've had mine fail (mag stripe wear, etc.) maybe 3 times in 20 years, but even then it's still usable the old-fashioned way. My iPhone fails (low battery, weak signal) at least weekly. Being occasionally unable to pay is a Really Bad Experience that users won't forgive.

Slower and less reliable. I predict it won't take off.

[mrtron]

Also problems with entering in the amount you are paying them instead of that being automated. What if you mispress and over/under pay by a cent, a dollar, 10$...?

The trick for any service like this is mass adoption. The service needs to be available in enough places so you don't need to carry your credit card. That seems unlikely.