There is a dedicated section about eager loading in the docs. A coworker discovered eager loading does not work in polymorphic associations the hard way so I've written a patch that documents it:
http://dev.rubyonrails.org/ticket/10610
There are at least a couple of tickets that implement something that would change that though:
http://dev.rubyonrails.org/ticket/9640 http://dev.rubyonrails.org/ticket/8119
What do you think?
-- fxn