The rest of OpenStack is just cloning the administrativia, honestly. Only the most rigorously scaled AWS app, for instance (I'm thinking about things like Reddit here), is going to need the programmatic ability to spawn EC2 hosts, etc... At the startup scale, that's just bandwidth you'd be better off spending on your product's value, not cost savings.
So, I'm all in favor of OpenStack. But I don't think that cloning Amazon's API is going to win anyone any business. You have to beat them on price and quality. We'll see.