It is not a secret ... I like Source Code Pro!
A couple of weeks ago I got myself a new MacBook Air (and resisted the temptation to get the 13-inch and like the 11-inch a lot) and installed Source Code Pro again. Getting it installed using FontBook was no problem, but then I could not remember how make it the default font for Emacs and all the tips and tricks that I was able to find did not work for me.
At the end the solution was simple: I am using GNU Emacs and to start it I am using the Terminal app, means I just created a new profile, made sure that the Meta-key works, changed the font to Source Code Pro and started Emacs from this window and voila ... I am in business.
PS: Ctrl-+ and Ctrl-- also works to increase/decrease the font size, in case you want to show source code on a screen during a presentation.