With this in mind, (price) scalable-CDNs are a good idea. Don't spend a lot of money up-front when you're not sure you'll get the traffic to merit it. I'm glad I didn't, because my site got all of five hits from HN... though, granted, it's not tech.
My google-fu is pretty weak, it seems. I'm not able to find any examples of how apps built on Ruby on Rails and deployed with Heroku were ensured to be robust.