There's a discussion over at AA forum about 12 of 16 F passengers being moved back to coach on a A321 (non-neo) to Hawaii for weight and balance issues due to a lightly populated economy section. Luckily, I haven't seen that on a UA plane to Hawaii (yet).
Post #340 (page 23) by MauiOC, with a lot of interesting follow-up discussion:
http://www.flyertalk.com/forum/ameri...idated-23.html