There were a bunch of threads on boarding methodology a few months ago, sparked in some measure by http://www.wired.com/news/technology/1,70689-0.html

https://www.flyertalk.com/forum/showthread.php?t=556825
https://www.flyertalk.com/forum/showthread.php?t=556546
https://www.flyertalk.com/forum/showthread.php?t=488796

There was also an ASU study and a Wall Street Journal article.

America West was cited as using the "reverse pyramid," which boards starting with aft window seats and fills into forward aisles, whereas United adopted "Wilma" (Windows-Middles-Aisles) years ago, and Southwest is famously open.