I had a similar paranoia, and it kept my device in good condition for about 3 good months, since I received the device. But it started to wane after that and in next few days, I managed to crack my tablet's screen. I have attached the photos.
@Paola: You have used some of the best words imaginable to describe the problem we all face - we wont really know the amount of shock/impact our devices can take, and thus, most of us will either be too paranoid or too careless. Mine is the second case, I had spent awful lot of time waiting for a reading device and when I finally received one, I didn't really bother much about getting enough protection and was busy doing the reading almost the whole time. The device went everywhere I went - work, grocery shopping, bus, train, bed. And it worked like a charm the whole time. But my clumsiness got the better of me one morning.
I had kept my tablet (8" apad) on the floor bound in its leather jacket below my bed just before I went to sleep the previous night, and I was picking it up to use in the morning. I had lifted one edge of the device to a height of about 2 inches from the ground, the other edge was still on the floor. My hand slipped and the edge fell back on the floor, and it was like a gentle tap. I had kept the tablet with its screen facing down. I had never even imagined that such a light tap would break the screen, but it did.
@Dixiegal: if you are placing the tablet on a hard surface like a table or kitchen slab, then be extra careful while picking it up from there later. The floor at my house is covered in polished marble and a hard surface like that can do some serious damage. That day was gloomy for me.