As long as it's machine-readable (with a barcode at the bottom of the photo page), all you need to do is fill in the visa-waiver forms which you will be given on the plane.
With reference to SteveD's comment, the US regulation regarding machine-readable passports (at least for UK citizens) has been delayed until October next year. See the following website for the text of the US Govt wire to its consulates, confirming this delay - http://travel.state.gov/visa/tempvisitors_novisa_waiver2.htm l
Sorry, scrub my last answer. I'm sure I've seen somewhere that the requirement had been delayed for a further year; but can't now find it. Best to assume that to enter the US under the visa waiver program, you DO need a machine-readable passport from October 2004.
Sylday, it was due to be introduced in 2003 but was postponed till October 2004.
From the Foreign & Commonwealth site:
"From 26 October 2004, all British passport holders, including children, travelling to the USA under the VWP will need their own machine-readable passports (MRP). From 26 October 2004, anyone without a MRP, including children who are currently on a parent's passport, will need a visa to travel to the USA."