I am actively looking for Senior Ruby developers, and after reading the recent discussion thread on the topic of senior/intermediate/junior, I'm afraid there seems to be some misunderstanding with regards to what does the word 'senior' actually refer to.
I think the confusion stems from the word 'Ruby' that wedges itself between the word 'senior' and the word 'developer'. Look at it this way: what would you think if you read an ad where someone is looking for a senior Black & Decker developer?
So please, when I say Senior Ruby (or Rails) developer, don't read it as an expert who is senior in his/her knowledge of Ruby, but rather as a person who is a senior in his/her knowledge of software development in the service of the business. I've seen programmers who can play so-called 'Ruby golf' till the cows come home and beat everyone else at that game, and yet I would never hire them on a project.
It's not the knowledge of Ruby that is crucial for your success as a developer, it's the knowledge of how to build working software that will be robust, secure, that will provide utility, make the delivered product habitable for both end users and other developers while at the same time delighting the customers. In any programming language.