Android 2.3 comes out in a few weeks, and it includes a new technology known as "Near-field communication". As far as I can understand it, it basically means that your phone will know where you are and what you want, so it will give you a steady stream of information. If you're in a CVS, it will present you with coupons…. that sort of thing.
Ultimately, it looks like Google, who creates the Android operating system, would like Android phones to replace credit cards… which seems entirely do-able in this day and age.