Have read another answer relating to this somewhere else on the site. This said that the EU privacy laws prohibited the police records being made available to the US immigration authorities. Depends what the arrest was for - things like drugs, terrorism etc mean that the privacy laws would not apply. I do know several people who have just ticked the no on the visa waiver form and got through OK. I think you would have to be a known criminal or on someones wanted list for them to know about you.