As someone who just 'picked up' a rails 3 app to add an online order form, I agree that, while you may be able to 'get stuff done', you'll be slower than someone who has experience, no matter how senior you are. At least for a few months. I banged my head against tasks that a RoR developer would have finished far quicker.

However, by being willing to train senior folks (and I'm talking my book here, as I am a senior developer), you gain two things: access to a larger pool of applicants, and the value of cross pollination. (I've seen a lot of ORMs in my time, and concepts translate between them.)

How to choose whether to train or not? It depends on the length of your runway (longer means you'll have more time, obviously), how many other current folks have knowledge of the solution (and can offer code review or other guidance), how committed the applicant is (tough to judge, but desiring employment vs wanting a contract is a proxy), how far you are pushing what you are building (if it is a normal use case--crud app for rails--a book and a few days may be enough training time. If it is not--high traffic erlang application--you may have a harder time training someone up without extensive partnering), and how hard it is to find someone who has experience with your current technology (there is an opportunity cost to training someone, but there is an opportunity cost to having an empty seat as well).

Certainly any senior developer worthy of the name is going to be able to pick up a language and be productive with it in a few weeks. However, they'll make mistakes, just like anyone will, that may require rewriting later.

Junior folks are a whole different ballgame.