From Jan 12 2009, travellers going to the US under the Visa Waiver Program (VWP) is now required to apply for an ESTA (Electronic System for Travel Authorization) before they board the plane. It’s a free application, where you are required to register your details online. Here’s a brief Q&A I got from the website:

• Who is required to apply for a ESTA?
  Coutries participates in the VWP are now required to apply for travel authorization, they are:
  Andorra, Australia, Austria, Belgium, Brunei, Denmark, Finland, France, Germany, Iceland, Ireland, Italy, Japan, Liechtenstein, Luxembourg, Monaco, The Netherlands, New Zealand, Norway, Portugal, San Marino, Singapore, Slovenia, Spain, Sweden, Switzerland, United Kingdom
• How long does the authorization process take?
  It is authorized immediately, if not, you will receive a response in 72 hours.
• How much does it cost?
  It is free (for now).
• How long is the travel authorization valid for?
  It is valid up to 2 years, if your passport expires before the 2 year period, then the travel authorization is only valid till the expiry date.

For more information, visit the US Customs and Boader Protection website.

So don’t forget to apply for ESTA before you travel, they can stop you from boarding without that travel authorization!