So, by adding EC2 instances that are all attached to the same S3 bucket, and using round-robin dns to distribute traffic among them, you can theoretically scale a web service infinitely. Just launch a new EC2 instance and add it to the round-robin dns pool and you're done! Even that could be automated to respond dynamically to traffic.
There's some additional information in the discussion thread.
Thanks to Jin for pointing me at the original blog post!