Ruby on rails for startups?

Ruby on rails is one of the hottest buzz words at this moment. Comparing R/R to the current J2EE and .Net. I would like to raise a question about the performance and scalability, since I have not found any industrial data yet.

Until the performance/scalability is proved, I treat R/R as a great tool for startup companies. Startup companies should be able to dive into their prototype quickly and raise the money based on the prototype. But what they are going to do next...is a secret:)