I've been trying to figure out how Ryan Bates, in his Facebook Authentication screencast, is setting the following "FACEBOOK_APP_ID" and "FACEBOOK_SECRET" environment variables.
provider :facebook, ENV['FACEBOOK_APP_ID'], ENV['FACEBOOK_SECRET']
There are similar-ish questions around, but no answers that I've been able to get to work on Rails 3.2.1.
UPDATE:
As of May 2013, my preferred way to handle ENV variables is via the Figaro gem