Could it be in a bag where something is putting pressure on the power button? I've read about it being pretty sensitive. I've also read about an issue where the power button recesses into the phone due to friciton with teh battery case. People have solved this by either taking it to VZW, or filing a tiny bit of material off the part of the back cover that touches the power button.
If the phone is sitting on the table by itself and turns itself in, there's a problem; take it to Verizon. I haven't heard of this issue before, so hopefully it's an isolated incident.