Rick, the problem on drywall is usually that someone did not seal and prime the wall prior to installing the wallpaper. This means that the wallpaper is glued directly to the paper on the drywall without a water barrier in between. When you put on the water to remove the wallpaper, the drywall paper is going to get wet too and that's where you have problems. You will not have the same problem with plaster. One tip for you though is that using a chemical like DIF or adding vinegar to the water will make your job go faster.