1. The Rehabilitation of Offenders Act does not apply outside of the UK so, as far as the US embassy is concerned, your conviction will never be regarded as spent.
2. Several AB contributors have posted to state that they've not experienced any problems entering the USA, under the Visa Waiver Program, with undeclared criminal convictions.
3. The information currently forwarded by the airlines only consists of 'routine' data, such as your nationality, address, etc, together with details of the debit/credit card which you used to book the flights.
5. EU privacy legislation prevents the UK government from making UK criminal records available '
en bloc'. They can only pass on specific information when there is 'justifiable cause'. (i.e. they can pass on information about suspected drug traffickers, terrorists or 'sex tourists' but they can't allow the US free access to criminal records).
6. The US authorities have no direct access to any criminal records, other than those of the USA and Canada (with whom they share a common database).
7. Under 'exceptional circumstances', the US immigrationservice will be able to access 'sensitive' data collected by the DHS, which might include your religion, sexuality, etc but still doesn't include criminal records:
http://observer.guardian.co.uk/uk_news/story/0 ,,2132099,00.html
Chris