Is it possible to shift the camera viewport?

Use the following script :

 function SetVanishingPoint (cam : Camera, perspectiveOffset : Vector2) {
     var m = cam.projectionMatrix;
     var w = 2*cam.nearClipPlane/m.m00;
     var h = 2*cam.nearClipPlane/m.m11;
  
     var left = -w/2 - perspectiveOffset.x;
     var right = left+w;
     var bottom = -h/2 - perspectiveOffset.y;
     var top = bottom+h;
  
     cam.projectionMatrix = PerspectiveOffCenter(left, right, bottom, top, cam.nearClipPlane, cam.farClipPlane);
 }
  
 static function PerspectiveOffCenter (
     left : float, right : float,
     bottom : float, top : float,
     near : float, far : float ) : Matrix4x4
 {
     var x =  (2.0 * near)        / (right - left);
     var y =  (2.0 * near)        / (top - bottom);
     var a =  (right + left)        / (right - left);
     var b =  (top + bottom)        / (top - bottom);
     var c = -(far + near)        / (far - near);
     var d = -(2.0 * far * near) / (far - near);
     var e = -1.0;
  
     var m : Matrix4x4;
     m[0,0] =   x;  m[0,1] = 0.0;  m[0,2] = a;   m[0,3] = 0.0;
     m[1,0] = 0.0;  m[1,1] =   y;  m[1,2] = b;   m[1,3] = 0.0;
     m[2,0] = 0.0;  m[2,1] = 0.0;  m[2,2] = c;   m[2,3] =   d;
     m[3,0] = 0.0;  m[3,1] = 0.0;  m[3,2] = e;   m[3,3] = 0.0;
     return m;
 }

Call OffsetVanishingPoint by passing a camera and a Vector2 containing the x and y amounts that you want shifted compared to the default vanishing point. The Vector2 is an absolute offset, not a relative offset. A sample usage snippet:

 // Do a pan from left to right
 var panSpeed = .15;
 var panLimit = .1;
  
 function Start () {
     var t = 0.0;
     while (t < 1.0) {
         t += Time.deltaTime * panSpeed;
         var x = Mathf.Lerp(panLimit, -panLimit, t);
         SetVanishingPoint(camera.main, Vector2(x, 0.0));
         yield;
     }
 }

Be careful, as the docs say, using projectionMatrix will make the camera no longer update its rendering based on its fieldOfView, so using this function will make the camera stick at whatever FOV it had before you used it. If you want to change the FOV after using this function, you have to call ResetProjectionMatrix first.

Link : http://wiki.unity3d.com/index.php?title=OffsetVanishingPoint

 Vector2 pixelOffset; // Your pixel offset lives here
 Camera camera; // This is your camera
 
 float scale = (camera.nearClipPlane*2 * $$anonymous$$athf.Tan((camera.fieldOfView/2) * $$anonymous$$athf.Deg2Rad)) / Screen.height;
 Vector2 perspectiveOffset = pixelOffset*scale;
 
 SetVanishingPoint(camera, perspectiveOffset);

After messing around with the previous script, there was a problem in the web player when the aspect ratio changed where the camera wouldn't adjust its perspective properly. Objects would be squashed or too thin.

I ended up implementing this and now it works as intended: http://unity3d.com/support/documentation/ScriptReference/Camera-worldToCameraMatrix.html

I wasn't clear on that before and wanted to post this answer in case it helps someone in the future.

So now my camera can shift its viewport, even animate the shift when certain gui elements are on the screen. It also properly displays the perspective when the web player is resized.

Thanks!

Chris