Would you like the 'official' answer or the 'unofficial' one?
The 'official' answer is that the Rehabilitation of Offenders Act doesn't apply to the USA and you must apply for a visa. The process is likely to take around 5 or 6 months. It will involve attending an interview at the US embassy and, for much of the waiting period, you'll effectively be barred from international travel (because your passport will be held by the US authorities).
The 'unofficial' (but still accurate) answer is that the US immigration service has no access to the criminal records of any countries, other than those of the USA and Canada. The UK is prohibited, under European human rights legislation, from making personal information (including criminal records) available to foreign governments without 'justifiable cause'.
If you're suspected of cross-border crime (such as drug-trafficking, terrorism or sex tourism) the US authorities will be notified of those suspicions (and of your criminal record, including unrelated offences). Otherwise, they'll know nothing about your past and you'll be able to enter under the Visa Waiver Program.
Chris