I'm no lawyer but I do have a decent understanding, and unfortunately I do have some familiarity with your situation (although not quite as much as I'd have liked!!!)
The term you use here that I must turn your attention back to is this lovely word "de-arrested". Now whatever you've read, been told or heard - you were not "de-arrested", you were very much arrested (on suspicion of driving/ attempting to drive with more than 80mg alcohol per 100ml blood), the Police were unable to provide sufficient and acceptable evidence to charge you and refer you to a magistrates court, and therefore had to release you without charge (hats off to your liver for getting shut of the additional 6 units before they got you to the station - was it a long drive?!?!) ***Did they take prints and DNA - just out of personal interest???
So in response to the essential query of your post, no one can tell you definitively either way what to declare on your Visa form, that decision will rest ultimately with you, but one thing is for sure, if the US Embassy does do a check then I'm afraid it will (as in should) show up on the Police database. How long an arrest without charge remains on the PNC I'm not entirely sure but from what I gather any information on the PNC can be held indefinately - if this is wrong someone please correct me!
My advice would be to declare it, and I'd speculate that, due to the specific circumstances of your case, there is an 85% chance of you getting your Visa without hearing anything of it. If you do hear of it, and it is proving a sticking point, I'd suggest you have MASSIVE scope for an appeal that any two-bit emmigration lawyer could handle in a fairly straight forward manner, with delay being the main inconvenience. Lets be honest, there are a number of indicative factors that suggest our cousins across the pond don't take DUI as seriously as ourselves (Their president being a convicted DD, the "Walk
straight along this
straight line painted on the floor in front of you, as opposed to roadside breathalysers)
If this is the only thing you've ever done wrong, then like I say (hopefully not building you up too much) I think you'll have no problems at all. I would suggest though that if you do have other history spent or unspent in terms of rehab of offenders, I'd declare it all.
*It maybe worth getting some professional advice before you go any further if funds permit, I'm sure it would only be about £100 tops to see a a specialist immigration/emmigration solicitor about this anyhow, which isn't OTT for peace of mind.
Whatever you do decide good luck and keep me/us posted for interests sake
