You could, in theory, use a Visa, MC or Amex-branded gift card.
Yes, you could... But at the same time, you could use Visa, MC, or Amex-branded gift cards in places other than the Android Market as well..
What gift cards like these do, is confine people to purchase from a certain place. Visa and MC gift cards, you can use most anywhere. An Android gift card can only be used for the Android Market.
Personally, I think an Android Gift Card would be a GREAT idea. 1) It would pump more revenue into the market, driving developers to create better/higher quality applications that are more worth people's money. 2) It would give developing companies and studios who have already created successful apps on other platforms more reason to invest time/manpower into creating more of the apps that everyone is missing/wanting on Android. 3) It would be much more beneficial to those creating apps, which is really what will help the market and the OS grow. Apps play such a pivotal role in the success and longevity of a mobile OS. The less money that continues to be spent in the Market, the less innovation, the less quality, the less diversity there will be; the more crap themes, the more crap "sexy pics" apps, the more pointless games we will see flooding the market.
Several people think that gift cards are only good for those who are too young to have a credit card to purchase apps. That's not the case. Sure, spending $20 cash for a $20 gift card to purchase apps would be used by those unable to use a credit card is very popular over on Apple. But a HUGE number of cards are also sold as gifts, or are given as incentives to purchase things.
If Google took the initiative, showed they were serious in supporting the developers, a gift card would be the perfect thing. According to some numbers, there are 300,000 Android activations occuring EACH DAY. If Google even did something like "On this certain phone, you will receive a $10 Android gift card after your purchase", that would be a HUGE amount of revenue that would be sure to hit the market in many different ways. All for the benefit of the community.
I really hope Google has this on their roadmap somewhere. Sooner than later, please.