It’s always fun to see this demonstrated. Here’s Haggis Henderson, at the recent NZ Institute of Physics conference in Christchurch, not only lying on a bed of nails but having a teenager stand on him too. He survived the experience, though I can’t vouch for what his back looked like afterwards.
The bed of nails doesn’t hurt if you lie on it gently. (I know, I’ve had a go before.) Although each nail is sharp, there are a great many in contact with you and so your weight is spread out over many nails. The pressure each nail exerts on the body is not enough to cut through the skin. (I wouldn’t recommend wearing an expensive shirt, however.) Probably the biggest danger is getting on and off, when you have less of your body in contact with the bed.