"We’ve effectively hired them to deal with managing the hard parts of our scaleability infrastructure and they are working for a fraction of what it would cost to do this ourselves."

This, in my mind, is the most insightful line in this post. When cloud hosts like Heroku and GAE are discussed on HN often there is cost comparison between using them and doing the sysadmin yourself on Amazon, or on your own hardware. But what doesn't figure in is that with Heroku and GAE (I suppose more with GAE) is that you aren't just getting out of doing your own sysadmin work, you are also getting out of doing a lot of scaling work yourself. This is expensive, difficult work. Of course it is the type of work that many of us here salivate over, but that's another issue... ;)