A caution is not a conviction and so it doesn't, per se, have to be declared to the US authorities. However, it was almost certainly preceded by an arrest which does (officially) render you ineligible to enter the USA under the Visa Waiver Program.
As is frequently stated here on AB, the US immigration service has no direct access to UK criminal (or police) records. EU law prohibits the UK from making the records available to other governments. If you simply 'forget' about your brush with the law, you'll have no problem entering the USA under the VWP.
If you're really worried, you can 'go by the book' and apply for a visa. Remember to allow plenty of time (at least 5 or 6 months). Remember also that you won't be able to travel abroad for most of the time while your application is being considered, because your passport will be held by US officials. Full details of the process, which involves attending an interview in London, can be found here:
http://london.usembassy.gov/cons_new/visa/niv/ index.html
Chris