I do it: Use RSpec(v1.10, since it support rbehave), I start by writing user story...
What about you
The first thing that I do when building a demo, or prototype, in Ruby on Rails is determine what I'm trying to do. If this is a project for an internal purpose, I try to identify what the problem is, and what the top needed features would be. Once I've done that, I can begin whipping up a database schema to reflect the data structures, and then start massaging in the basic functionality. After that, it's back and forth refactoring of the database and the codebase.