Further to Booldawg's post:
Officially (i.e. according the to the information on the US embassy website) you're ineligible to travel under the Visa Waiver Program and you must obtain a visa. Since it usually takes two or three months just to get an appointment for the interview at the US embassy, you've absolutely no chance of getting a visa for Christmas travel.
So, as Booldawg suggests, you'll have to simply keep quiet about your conviction. At present, you can simply fill in a form on the plane (while, of course, conveniently 'forgetting' about your conviction). Alternatively, you can complete the procedure online, prior to travel. (From 12 January 2009 this method will be compulsory). Whichever method you use, don't mention your conviction as to do so would be admitting to your inelegibility to travel without a visa, which would automatically bar you from entering the USA.
For further information, check out the links in the 'ESTA' box, at the top right of this page:
http://www.usembassy.org.uk/
Chris