Like you said, I get people to use Sinatra for a while before getting in to Rails. It gets them in to MVC style apps while keeping things simple. Definitely recommend it as a stepping stone to Rails.
I don't think you need to go from Sinatra to Rails - I actually started with Rails and like dhh said - before I knew it I was building real apps. Rails provided a great framework to get something up and running. It provides MVC and interacts with a database. After familiarising myself with Rails I started using Sinatra (which doesn't offer any MVC or support you get from Rails) and then ultimately started coding just in Ruby. I don't think there's any issue with the learning curve for Rails because it provides the support where you need it. I speak to new developers all the time who use Rails and get it almost instantly - releasing apps a week or two later.
micdijkstra|14 years ago