If you’re travelling to the States, you need to get an ESTA.  Visit USA sent out the following advice:

ESTA MANDATORY FROM JANUARY 20, 2010

The position relating to the ‘grace period’ of 1 year for full compliance with the ESTA requirement has now been clarified and the date for implementation has    finally been resolved.

US Customs and Border Protection have advised that with effect from January 20, 2010 it will be mandatory for all passengers travelling to the USA under the Visa Waiver Program to receive approval to travel using the Electronic System for Travel Authorisation (ESTA) scheme. All passengers travelling to or transferring through the US from this date are advised to apply for an ESTA at least 72 hours in advance of their journey.

Under the new regulations, US Customs and Border Protection will not now be able to accept US-bound customers for their flight if they have not received ESTA approval or if they do not hold a valid visa or Green Card.  Although not fully implemented during the past year, most UK-USA travellers are already familiar with the ESTA requirement.

ESTA can be obtained free of charge by visiting the Visit USA Website at www.visitusa.org.uk or https://esta.cbp.dhs.gov where further information on the scheme can also be found.  Once completed & approved, the ESTA application is valid for two years and allows multiple visits without the need to reapply. Travellers can apply for an ESTA even if they have no firm plans to travel to the US.