I started with Rails. Though I had some strong foundations in OOP and programming principles in general when diving into RoR.
And I found out that I agree with the idea that you don’t need pure Ruby knowledge beforehand to learn Rails fast and effectively. Pure ruby will come into play when you need to code more advanced stuff. You will have decent knowledge of Ruby if you are good at Rails. It just comes naturally, because not everything that is needed FOR Rails app is written IN RoR - sometimes, it’s just pure ruby.
IMO, just pick one of the better beginner/tutorial books that advertise “Ruby on rails for beginners”, and keep checking the API documentation for both Ruby and RoR when you stumble upon something you don’t understand. This way you will learn what you need to do things at hand fast, and the knowledge will build up eventually, for both RoR and Ruby.
I advocate this way also because Ruby is far more general than RoR. RoR is pretty much about Web or anything involving browser, while pure Ruby can be used as sysadmin language of choice. I think you see where I am going with this: it’s better to learn the stuff that you are going to work with/on (i.e. RoR is far higher priority than Ruby when starting out).