This is why I insist people to work on a robust stack from ground up, so it will be less painful in the long run. This is not to say that you shouldn't build a prototype in Rails to get everything up and running as quickly as possible and worry about scaling later, but, it is just my opinion that if you invest the time and effort in working with a robust stack (for example Scala+Lift), your investment will pay you off in the long run.

I've always admired rails for its flexibility and its enormous productivity boost, but all my serious applications are coded in Lift. I for one believe in "develop and forget", because I'd like to call myself a business guy than a programmer, though I'm deep into both. I like to spend more time expanding/marketing my business than worry about scaling it. But that's just my perspective.

JVM is terribly under-estimated and I realized this when I got started with Lift+Scala. Scala is a very powerful language and requires a totally different mindset (=functional). And Lift is fairly complex for those wishing to get started with it and has poor documentation, despite being a 5-year old Framework. But once you understand it fully (somehow), there's no looking back. Lift provides so many things out of the box, especially related to security (unlike PLay!), so it's kind of a trade-off you have to choose between. Even if you compare all the benchmarks, most of the JVM-based languages like Scala outperform even something like GO! (Ok, that's not fair, since GO is fairly new)

If you're interested in Scala, Coursera has a course on it by the creator of Scala himself (Martin Odersky).