Sniper Scope See through

How would I make a scope that has a see through cross-hair reticle in the place of the texture the mouse is on?

Thanks in advance.

DISCLAIMER: You can only do this with Unity Pro.

One way to do this is to make a render texture attached to a camera pointing out the front of the scope. This will not look quite like a true lens- it will act more like the viewfinder on a digital camera. With a specialised view matrix you could improve on this, but be prepared for some fairly confusing mathematics.