Just wanted to add my opinion that yes, learning Sinatra or Padrino will definitely help you learn Rails. In the sense that it will make the transition from PHP (or whatever) to Rails a lot less daunting. As wuliwong said, Sinatra and Padrino are way less complex than Rails. In fact, Rails is too complex for its own good in my personal opinion, routing in Rails is a bit of a nightmare, but let's not get sidetracked.
Also, while it is true that you cannot learn Rails without learning Ruby, there is no reason whatsoever why you couldn't learn Ruby as you lear Rails (or Sinatra/Padrino), it's a fun way to learn the language.
I started with Rails directly, with no knowledge of Ruby at all, coming from PHP. Did a lot of the tutorials out there, bought a few books and kept slogging on. After about 7 months of work with Rails I moved to Padrino to build an API and immediately loved its simplicity compared to Rails.
Once you get good, Rails has a lot of high-end magic that is awesome, but while you're starting out it's very overwhelming. Sinatra/Padrino allow you to start out smaller, easier and keep adding.
Good luck!