I had a Stearns Foster and had one of those inspectors come out - TWICE - with that string test. The second time, I convinced him to warranty the damn thing. I had such back problems from that mattress that it would take about an hour to get straight in the morning. The inspector said to me that of course my mattress sags in the middle, it was because I was single. Huh?
Now I have a tempur-pedic and absolutely love it. It's been a couple years, I got it when I pulled my psoas and really needed something supportive. I like this particular model because it's got bamboo or something and so it doesn't generate as much heat.