hire enough engineers and buy enough hardware that scaling won't be an issue for long

I don't know about that. Twitter is still having problems with downtime due to scaling. The way I see it: don't completely ignore scaling, but don't focus on it (at all) at the beginning. Keep it in your mind though, and try not to close yourself in, and create something which can't be scaled without significant modifications to services. Do not confuse that with modifications to your code. If you're going to scale large, expect at least one complete re-write, but don't offer services which are incapable of scaling.