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)

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour 
{
    void Update() 
    {
        Vector3 CameraCenter = camera.ScreenToWorldPoint(new Vector3(Screen.width/2, Screen.heigh/2, camera.nearClipPlane));
        if (Physics.Raycast(CameraCente,  transform.forward, 100))
            Debug.Log("Ou yeah!");

    }
}

p.s.

I didn't test this code ;)