Passport or no?

I thought I was going to have to spent $300 to get passports for my
wife and kids when coming back from Mexico next year but now I'm not
sure. Reading AOPA's site
http://www.aopa.org/whatsnew/newsite...port_info.html
They claim that after Dec 31st "will be required to have a passport or
other acceptable document". However, at the top they say "submit a
passport or other acceptable documentation as proof of citizenship".
So, this makes it sound as if kids can continue to travel with just a
birth certificate. Am I missing something? I certainly don't want to
pay $300 for no reason. I'm also pretty sure that customs will never
actually ask for it since they've never asked any of us for citizen
information in all the years we've been traveling to/from Mexico and
Canada. The worst I've ever been asked for was my driver's license and
pilots certificate. My wife was asked for her driver's license once
(and illegals can even get those!).
The actual rules and what the individual custom's person do are usually
different.

-Robrt