More and more people just say Rails instead of Ruby on Rails nowadays... and isn't it Rails, which is developed on top of Ruby, so it is really Rails on Ruby?
Update: when people say: I am using Rails, that means it is Rails they are using. When people say: I am using Ruby on Rails, that means they are using Ruby that happens to be on Rails. The focus is not on Rails but on Ruby. So technically, people don't actually mean they are using Ruby per se. They mean they are using Rails.