my advice would be to paint over it, this is the easiest way, and if it has a pattern on, it should show up through the paint, it has quite a nice effect,
You could try TSP - it's the solution you use before painting walls. Don't know if it will harm your wallpaper tho. Other thing that might work is Simple Green - which is a non=toxic all-purpose cleaner.
I am going to give you a homemade cleaner recipe that should work. You can use this on floors and walls. Give it a try!
Fill a bucket with about 1 gallon of warm water.
Add about 1/3 cup of Borax. (20 mule team Borax sold in laundry isle at many stores)
Add 1 tsp liquid dishwashing detergent.
Mix inn about 1 Tbsp. Ammonia.
Stir well, until Borax dissappears.