I was surprised - in that I didn't see it coming, but I don't think it was at all unbelievable given everything that had happened with the Dukat character up to that point. I definitely think hiding it from the audience until the moment it was dropped on our heroes was the way to go. It stands out as one of the great "holy &%#$!" moments, in a show that - as ryan123450
alludes to - was full of such moments.