Not all nationals of VWP countries may enter the United States without a visa.
To enter the United States under the VWP, you must:
VWP entrants are screened prior to admission to the United States and must participate in the US-Visit Program. and may not extend their stay in the United States beyond the 90-day period.
U.S. Dept. of Homeland Security - Visa Waiver Program Traveler Guide (PDF)
Visa Waiver Program Emergency Passport Requirements (PDF)
CRS Report - Visa Waiver Program - Jan. 24, 2006 (PDF)
U.S. Dept of State - Visa Waiver Program
U.S. Customs and Border Protection - Visa Waiver Program Passport Requirements
U.S. Customs and Border Protection - Visa Waiver Program Applicant Responsibilities
U.S. Customs and Border Protection - Electronic System for Travel Authorization (ESTA)
U.S. Dept. of Homeland Security - ESTA
U.S. Dept. of Homeland Security - Visa Waiver Program: Passport Requirements Timeline
USCIS - WB Temporary Business Visitor under Visa Waiver Program
Shaping a Visa Policy in the National Interest - Washington Post - Feb. 5, 2007