What's an efficient way for someone who knows PHP and Ruby on Rails to quickly pick up the Zend framework?
I think that by already having experience with RoR and PHP, you have already broken the back of the task. RoR follows the same architecture pattern as the Zend Framework (MVC); and so you will already have an abstract understanding of your first Zend Framework application.
I don't know of any resources that are specifically targeted at transitioning RoR programmers to Zend Framework programmers (that would make a good topic for a book), however, with (at least) a syntactic understanding of PHP, I'd recommend you read one of the following introduction articles:
You can always post questions on S.O. if you're stuck!
I'll add the nabble forum dedicated to ZF that helped me a lot.
I'd also like to point out the FREE Zend Framework Book found in: http://www.survivethedeepend.com