What's the Rigidbody's gravity unit?

You should then know that mass is not a factor : In the fourth century B.C., Aristotle maintained that an object falls with a speed proportionate to its weight that is, the heavier the object, the faster it falls. This idea was generally accepted until the 17th century, when Galileo showed that the rate of fall caused by gravity is the same for all objects.

http://science.howstuffworks.com/falling-bodies-info.htm

anyway, in regard to your problem, I just did a test and I think you are correct! Here is my script, attach it to the rigidbody, set the rigidbody inspector to use gravity = false, is$$anonymous$$inematic = true. When you click the L$$anonymous$$B then is$$anonymous$$inematic is disabled and gravity is enabled, the object falls and the timer starts. When it hits something, the timer stops :

 #pragma strict
 
 var rigid : Rigidbody;
 var isTi$$anonymous$$g : boolean = false;
 var timer : float = 0.0;
 
 function Start() 
 {
     if ( !rigid )
     {
         rigid = this.gameObject.rigidbody;
     }
 }
 
 function Update() 
 {
     if ( Input.Get$$anonymous$$ouseButtonDown(0) )
     {
         rigid.is$$anonymous$$inematic = false;
         rigid.useGravity = true;
         
         isTi$$anonymous$$g = true;
     }
     
     if ( isTi$$anonymous$$g )
     {
         timer += Time.deltaTime;
     }
 }
 
 function OnGUI()
 {
     GUI.Box( Rect( 10, 10, 100, 35 ), "" + rigid.velocity.magnitude.ToString() );
     GUI.Box( Rect( 10, 55, 100, 35 ), "timer " + timer.ToString() );
 }
 
 function OnCollisionEnter( other : Collision )
 {
     isTi$$anonymous$$g = false;
 }