I think a good question to think about is why someone would want to design an interview to specifically hire Ruby on Rails programmers. Other than a few very rare situations when you want an expert fast, what would a rails programmer offer that a C++ and Lisp programmer with a solid grasp of data structures and algorithms wouldn't? Even in the few cases when specialization is important, I still wouldn't design an interview entirely around Ruby on Rails. Every framework has some gotchas that you only learn with experience, but beyond that there is a ton of other stuff a good programmer should know that's outside the realm of any given framework, or even any given language.

I've seen programmers that were hired specifically for C++ positions struggle to move to J2EE projects, and it wasn't pretty. It's usually not a good idea to hire one trick ponies.