Ah, I didn't know that. Sorry. (I don't remember where I heard about single-AZ but it seemed consistent with observation)
Is Heroku resilient against single-AZ failure (so only some subset of customers go down, and then it restarts), or is it exposed so that if any AZ goes down, core stuff also goes down? The sites I care about on Heroku seem to go down whenever any US-East badness happens at all, even if it is "limited to a single AZ" per Amazon.