The condensation from your showers and baths will eventually cause your wallpaper to peel, if you have it in your bathroom. So, it's a big no-no in my book. That is, unless we're talking powder rooms where you're only running the sink; in that case, I wholeheartedly approve! It's better to get playful with paint, like the stripes I painted in my bathroom. Do you have wallpaper in your bathroom? If so, has it peeled?