I originally thought that one had to know both C# as well as UnityScript to develop using Unity. But it now seems that I only need to know one. So,
From my very limited development in Unity:
It's an either or thing.
I've written quick test-of-skill games using only C# as I'm more familiar with it from previous knowledge of C and C++. While reading the docs to decide whether or not I would try to port code over to the UnityScript code (again, as a test of skill), it became my understanding that either way you get the end result you want.
In these games I wrote all sorts of components, including UI, game object, terrain collision, even fiddling with jumping and the gravity engine. Never once did I have to stop and go "well darn. Looks like I need to use Unity Script for this."
Games in Unity, as far as I am aware, can use both. But I've never been forced into having to use one or the other.
If you ever run into a part where Unity forces you to use both, go ahead and come back to this post to yell at me.