I consider van Nistelrooy as well as most Dutch players to be a very intelligent and knowledgeable about the game. They know the rules or should do. They know that if the ball goes out to the wing and Ruud is not interfering with play or a player, that he just needs to move back to be onside.
So was it a set plan or was it an accident that Ruud was standing where he was at that time. Did the player who made the initial pass realise that Ruud was offside and therefore played the ball to another player, or was it a set plan worked out in practice.
Personally I don't think it was a specific set plan. I think Ruud was where he should have been, but was offside if the ball came to him. It didn't and he was definately not interfering with play (under the current rules) in anyt way shape or form.
Not offside when he scored. I hope that all makes sense :-)