Taken from an example from the docs: http://unity3d.com/support/documentation/ScriptReference/WWWForm.html

function UploadPNG() {
    // We should only read the screen after all rendering is complete
    yield WaitForEndOfFrame();

    // Create a texture the size of the screen, RGB24 format
    var width = Screen.width;
    var height = Screen.height;
    var tex = new Texture2D( width, height, TextureFormat.RGB24, false );
    // Read screen contents into the texture
    tex.ReadPixels( Rect(0, 0, width, height), 0, 0 );
    tex.Apply();

    // Encode texture into PNG
    var bytes = tex.EncodeToPNG();
    Destroy( tex );
}