According to the rules stated on the US embassy website anyone who's ever been convicted of any offence (except minor motoring matters, dealt with by a fixed penalty) is permanently ineligible to travel under the Visa Waiver Program. So a visa application is required. It's costly (US$131 plus an additional fee for the courier service to return your passport), inconvenient (involving getting hold of a copy of your police record and attending an interview in London) and usually very lengthy. (Typical processing times are between 3 and 6 months. The longest wait that I've seen reported, here on AB, was 10 months).
Most people around here would probably advise you to simply 'forget' the conviction and travel under the Visa Waiver Program (by applying for an ESTA):
http://www.usembassy.org.uk/cons_new/visa/niv/ esta.html
The US authorities don't (at least in theory) have open access to UK criminal records, so it's unlikely that the conviction would be picked up. However, a great deal of information is passed between the UK and the USA under the guise of 'anti-terrorism', so nobody really knows exactly what information the US immigration service are aware of.
Chris