I've had cardboard on my doors for a year and a half now. It works fine.
The idea behind sealing up your door is two fold: make a more solid area for a mounting surface and seperate the front/rear waves. Now, for seperating waves it doesn't matter what you use; as long as it's not acoustically transparent.
The first reason isn't near a big deal as some would like to make it and the cardboard is much stronger than people think. Cardboard density and steel density are pretty dang close. IMO, the use of cardboard has such a benefit that it more than merits its use in this application.
If anyone wants to see for yourself, come to North alabama. //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif