Since the beginning of this year you now need to get travel authorisation online to use the visa waiver system. I know some people have said this blocks the loophole of simply not telling the authorities about your past misdemeanour. However, I'd be very surprised if the UK have allowed the US open access to our criminal records. In fact I'd be astounded that it hasn't made the press. This question comes up regularly and I have never read of anyone who has actually been refused entry when simply 'forgetting' to tell of their convictions - but i've read dozens who've said they gained entry no problem. This applies both pre and post the new online system.
You'd therefore be very unlucky not to make it through and as the previous person says, if you apply and get refused thats it.