Somehow they should work it out so that a credit card is not required. Google keeps spam under control by verifying real people through SMS.
Be it $400, $100 or $50, AWS could (in my opinion should) go that extra mile and offer some option without requiring credit cards if it truly wants to take on GAE in this regard.
Only after much clamoring by people who actually wanted to pay Google money for more usage did they offer a way to pay for higher quotas, almost a year after the initial rollout.
I'll probably get an Android phone sometime soon enough, though. But I tell myself that the longer I wait, the more advanced it will be!