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.
My personal theory on why lightsabers deactivate upon being dropped, either via being disarmed, killed or knocked unconscious is due to the forging of the weapon.
Each Jedi creates their weapon personally, it's a final test to determine if they can wield the force properly.
When a jedi creates their weapon, they have to meditate and focus on the weapon, often building it by using the force to lift and connect the pieces.
This is illustrated by the creation of lightsabers at the forge on tython in the JK/consular prologue, where they use the force to assemble their first lightsaber.
When this happens, the force "binds" the weapon to the jedi and it becomes more then a weapon or a tool, but a piece of the jedi themselves, and they it.
When in combat, the natural flow of the force being used by the combatant allows their weapon to be more accurate and powerful and the weapon becomes attuned to that jedi.
When the weapon is dropped, or the flow of the force from jedi to weapon ceases, so to does the weapons power supply.
Not necessarily that the weapon is powered by the force, cause it isn't, but the sabre has a built in fail safe with their connection to the jedi and will de-activate when the flow is stopped.
While jedi can use lightsabers belonging to other jedi, the jedi will always attempt to make a new lightsaber as soon as they are able to if their original one was lost - Luke created his own sabre when he became a jedi.
Vader made a new saber when his was taken by obi-wan, Jacen Solo made a new saber when his was taken by luke and Anakin Solo even rebuilt his saber when his was destroyed on yavin during the Vong War.
It even described in the book how while the components were there, a jedi was not truely ready to wield a saber until they had meditated in the force on it.
Anakin Solo's unique choice of a lament crystal and the meditation afterwards even allowed him to sense the Vong in the force by channeling his senses through the crystal int he heart of his weapon.