Let's be honest, when you're starting a startup, Ruby's performance won't be a bottleneck until much later, when you're successful and get tons of usage - at that point you can afford to hire someone to fix it. Your productivity will be a bottleneck from the very beginning.

Pieter Levels writes his startups in PHP and hasn't had a performance bottleneck so far. For most applications, the performance of the language won't be an issue. I personally wouldn't pick PHP for any of my own projects, but Ruby I'd pick any day.