"If you're curious, the stack is Rails + Ruby Enterprise Edition + Passenger + nginx, which isn't cheap like PHP to host. On the $40 Linode right now but maxing out CPU at 350%."

Interesting. Makes me think that an MVP should be done in PHP or something else that is cheap to host and handles these sorts of conditions well, and then if things look bright build the real site in whatever seems appropriate for the expected demand.