I've got a hairline crack in a plaster ceiling and walls. The crack runs the entire width of the room and extends down the wall on both sides. I've tried spackling over the crack but this doesn't work. Most of the books I've consulted talk about fixing plaster holes; I haven't been able to figure out how to fix the crack. Can I fix this myself? What do I use? Any help would be greatly appreciated.

We recently bought a house, built 1950's in Tucson, Arizona. One of the walls in the master bedroom has a lot of cracks. The previous owners tried to repair them, but the cracks re-appeared. The wall is not an interior wall, and the cracks appear to be cause by the settling of the house. We cannot tell what the wall is made of (plaster, etc). My questions are: how do we determine what the wall is made of? Should we attempt to repair the cracks ourselves, and if so, what do we have to do? OR should we call in a professional? A friend of ours had a similar experience and just put new drywall up over the existing wall - is that a safe and viable option?