All documents from the United States require that an apostille be attached to them. These certify the documents according to international law.
A couple of weeks ago, I got apostilles for my sons' birth certificates in downtown Nashville. Today I sent off an apostille request for our marriage certificate from NY State (http://www.dos.ny.gov/corps/apostille.html). This is a trial run to see how quickly the documents are returned. I'll be sending off apostille requests for a number of birth and death certificates in the coming weeks and months as I receive them.