The unfortunate moral of all threads like these:
Birth certificates were
never intended to be a form of portable poof of citizenship (which is why pain-in-the-you-know-what issues with them like this come up again and again and again and again

).
Only
one thing is 100% inarguable proof of citizenship specifically designed for travel.
It's called a
PASSPORT
And the days when one could be casual and get away with traveling outside this country without one are ending, people.
Back on September 5th, the U.S. State Department revised the original proposed "Western Hemisphere Travel Initiative."
The proposed rules will require ALL U.S. and Canadian Citizens to carry a unexpired passport for travel to/from areas that were previously exempt.
This includes children and infants.
The proposed implementation is as follows:
December 31, 2006 - Passport
required for all air and sea travel to or from Canada, Mexico, Central and South America, the Caribbean and Bermuda. Note:
This will affect guests traveling on cruises that return to the U.S. after 12/30/06.
December 31, 2007 - Passport
required for all land border crossings as well as air and sea travel. Note:
This will affect guests traveling on cruises that return to the U.S. after 12/30/07.
You may print a passport application or get additional information from the government on this subject by visiting:
http://travel.state.gov/passport