Adding to this lovely discussion: Aren't lightsabers powered by the force? which would explain why whenever you see a jedi/sith defeated his/her lightsaber always sheaths. So really there is no chance of it going off while on you belt.
(someone may have said this, didn't read all the posts)
Incorrect. Virtually all lightsabers have a button, switch, or activation stud; however, they are almost always hung from the belt in such a way that the activator is less likely to be bumped to the "on" position. As to the second point about deactivation on death of the wielder, my theory is that most lightsabers require constant pressure on their activator to maintain the blade, or have a mechanism to "lock" the blade - for example, when the weapon is to be thrown.