Joshua posted more info on their setup in the comments section of the blog post:
Joshua Sierles 28 Nov 08
Our custom image is based on the Ubuntu Intrepid images from Alestic. We install useful EC2 gems and base packages, bundle, then provision each instance by role. Working with EC2 is so easy, we don?t see much value in using a third-party provider for our scale.
EBS and Elastic IPs were the primary motivation for moving to EC2 . We use EBS extensively: for MySQL data, logs and local repository mirrors. Performance so far is excellent. Snapshotting volumes is a breeze and makes setting up MySQL slaves and staging environments really easy.