If this is at runtime, check these two events out:
http://docs.unity3d.com/Documentation/ScriptReference/Collider.OnCollisionEnter.html
http://docs.unity3d.com/Documentation/ScriptReference/MonoBehaviour.OnTriggerEnter.html
Either of those should be relatively straightforward to implement and one of them might provide the functionality you need. The pages have script examples included as well.