World position of a mouse click with a tilted camera

In this 2D (2.5d?) scene, I have a camera at 0,-20,-25 and tilted along its x-axis to 310 degrees. I'd like to be able properly get the XY coordinates of a mouse click to eventually move the player to that point.

At certain points, where the user clicks is actually a non-intuitive world position due to the tilted camera, usually with Y movements. So the actual world position would be quite a bit off from where they clicked.

Any advice on handling this sort of situation? My current code is below, but not very intuitive.

        Vector3 dest = Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, Input.mousePosition.y, 0));
        dest.y -= Camera.main.transform.position.y;

For example: http://www.youtube.com/watch?v=Wg3DRwPsd0g

That looks like a 2.5d camera, but they somehow have the ship go to where the player clicks.