The United States Customs and Border Protection (CBP) issued a news release announcing that there is NO REQUIREMENT that travelers show proof of H1N1 Flu vaccination. No such vaccination requirement exists.
The CBP issued this release as a result of several rumors related to U.S. entry and the H1N1 virus.
The CBP encourages travelers to visit Flu.gov for tips on seasonal flu prevention, and cpb.gov for helpful traveling tips.