I think that best way to do taht is to use raycasting.
Raycast script reference: http://unity3d.com/support/documentation/ScriptReference/Physics.Raycast.html
Use ScreenToWorldPoint function to get the center of the camera. Your argument for ScreenToWorldPoint should be Vector3(Screen.width/2, Screen.heigh/2, camera.nearClipPlane).
ScreenToWorldPoint script reference: http://unity3d.com/support/documentation/ScriptReference/Camera.ScreenToWorldPoint.html
Example: (Attach it to an object that has a camera component)
I didn't test this code ;)