A few weeks past I asked for advice on cutting an opening in a plaster wall. Well, I got around to doing it today. I was going to use the cold chisel method recommended by Doug Miller, but I decided to try something else first. A couple of days ago I was looking for something and came across the scoring tool I used for cutting cement board. At the time I thought that this might work for cutting plaster. So, today I laid out the lines (it's about 2' x 2.5') and using a cork-backed straight edge, I proceeded to score the plaster. Worked like a charm. Once I got the score line going, I didn't need the straight edge. There was a minimum of dust and I only broke out one piece of the finished plaster (but in a place that will be covered by a face frame).
So, thanks again to all those who responded to my original post.