Sir Graham, to answer your question the bonus dvd was simply a video. sort of a promo talking about how the game was made. It was rather short not much more than the length of a movie preview. I would not go out of my way to get it or feel cheated if the US version does not have it.
I now have the game with the bonus DVD, and this claim is simply not true. The bonus DVD is very important since it contains the 5 minutes intro to the game. This intro CANNOT BE FOUND IN THE GAME ITSELF, so this DVD is not important only to people who don't watch intros...
SirGraham,
If you look at the North American version that InteractCD is selling, you can see that it mentions the bonus DVD on the front of the box shot (at the bottom) as being included:
LIMBO OF THE LOST BOX SHOT Hmm... Did you notice it says PC-CDROM instead of DVDROM? Odd.