The price levels of everything except for running an instance for an hour have dropped significantly since launch. This includes bandwidth (the pricing tiers used to be $0.18/$0.16/$0.13 per GB and have since fallen to $0.17/$0.13/$0.11 with the addition of a fourth tier at $0.10/GB), S3 storage & requests (used to be $0.15/GB for storage and $0.20/GB for requests, now is $0.12 - $0.15 for storage and $0.10 - $0.17 for requests), Simple Queue Service (the changes here are more complex but Amazon writes "Under the new plan, 76% of customers with bills greater than $1 would have received lower bills, saving an average of 71% each compared to their actual bill", so it's generally much cheaper), Simple DB storage (used to be $1.50 per GB-month, now is only $0.25 per GB-month), even the Cloudfront CDN has already gotten cheaper since its very recent launch date (the lowest priced tier used to be $0.09 per GB and they've since added tiers from 5 to 8 cents per GB).

So while you're right that the instance-hour cost has remained flat since the launch, the actual costs of using the AWS platform as a supercomputer or server farm has fallen dramatically.