This is from the US Department of State website http:/travel.state.gov/travel
December 31, 2006-Passport or other accepted document required for all air and sea travel to or from Mexico and Canada.
December 31, 2007-Passport or other accepted document required for all air, sea and land border crossings.
I don't know what the other "accepted document" would be.
My wife and 3 of her friends went to Puerto Vallarta last month and one of the ladies just had a birth certificate with an original seal (copies not accepted). I've been told birth cerificates will not be accepted starting December 31, 2006.
So the way I understand it, an original birth certificate will still be good this year, but a passport will be required for even kids starting next year.
Marty
PS when you go to the website there is special information for obtaining a passport for teens. Just look on the right side of the screen.