As of January 12, 2009, all nationals and citizens of Visa Waiver Program countries will be required by law to obtain a travel authorization prior to initiating travel to the United States under the VWP. You can access this authorization online through the Electronic System for Travel Authorization (ESTA), which is a free Internet application administered by Homeland Security (DHS). You can access the free application at 

https://esta.cbp.dhs.gov

For more information on the ESTA advisory statement:
Read this press release from Homeland Security