finally applied this 'getGravity' patch, so getGravity matches setGravity.

Thanks to Evgeny for the report/patch:
http://www.bulletphysics.com/Bullet/phpBB3/viewtopic.php?f=9&t=1852
http://code.google.com/p/bullet/issues/detail?id=23

src/BulletDynamics/Dynamics/btRigidBody.h

@@ -48,6 +48,7 @@ class btRigidBody  : public btCollisionObject
 	btScalar		m_angularFactor;
 
 	btVector3		m_gravity;	
+	btVector3		m_gravity_acceleration;
 	btVector3		m_invInertiaLocal;
 	btVector3		m_totalForce;
 	btVector3		m_totalTorque;
@@ -181,7 +182,7 @@ public:
 
 	const btVector3&	getGravity() const
 	{
-		return m_gravity;
+		return m_gravity_acceleration;
 	}
 
 	void			setDamping(btScalar lin_damping, btScalar ang_damping);