2d point to 3d point

I have coordinates such as (100,250) and (500,450) that I want to render a line between based on the 2d coordinates of the screen. How would I convert these points into 3d points that show up at the 2d coordinates for the camera or is there a way to bypass this conversion?

http://unity3d.com/support/documentation/ScriptReference/Camera.ScreenToWorldPoint.html