Ford found that an overly large plug gap eroded/damaged the plug electrodes on iridium plugs, this then increased the gap to a point where it put the coil pack under too great a strain and destroyed them. At dealer level the mechanics started setting the plugs at 1.0mm and this seemed to alleviate the problem to a great extent however ford had instructed them to use copper core plugs while they investigated the problem which was initially of unknown origin and thought to be a Mondeo/Duratec issue and initially spent time investigating the coil packs themselves as the weak point.
It was really the fact that the iridium plugs electrodes were being eroded to nothing very quickly as reported by dealerships that Ford thought it was an issue with plug quality and because dealers were replacing iridium plus at a now suggested 5000 mls it quickly became apparent that knock off plugs from china which certainly weren't iridium were infiltrating the market and the larger gap and increased spark to a small electrode had a predictable outcome. in the end this pointed them to plug gapping rather than coil pack issues.
Personally I set my plug gaps at 33 thou. with iridium plugs (2.0Lt Duratc). I visually checked spark consistency until I was happy as I reduced the gap and I have had no electrode wear at all. If you think you have found a bargain when buying iridium plugs you probably haven't and it may just cost you a coil pack!
If memory serves me well I think most iridium plugs are pre-set at 39thou.
