The Siege Specialist skill specifies that models with it only set off a mine on a roll of 6. I don't think there is any debate that this works against Castellan missle minefields but when you consider an Iron Warrior army, it changes things a bit.
To quote the Chaos Codex Page 41 "every model will automatically have the Siege Specialist skill". This overrides the usual restriction of only infantry being able to take it. So what happens when an Iron Warriors Predator drives over a Castellan minefield? By my reading, rather than being hit to rear armour on a 4+, it is only hit on a 6.
What say you?