Thanks for the reply.
The change you're referring to is the introduction of the ESTA (Electronic System for Travel Authorization) which requires all travellers from the UK to the USA (other than those who hold visas) to seek online authorisation to travel at least two days prior to travel. That obviously gives the US authorities more advance notice of who's arriving than the old system did. (Airlines used to forward the relevant information immediately after the plane had taken off). The introduction of the ESTA, together with fingerprinting of all visitors to the USA, led to a widespread belief that it would be impossible for anyone to enter the USA by simply 'forgetting' about their criminal convictions. That now seems to be untrue but, because information about what is passed from the UK to the USA is hidden under the cloak of 'counter-terrorism', nobody really knows for certain who'll be able to 'sneak in', and who won't.
Chris