longhorn-version.el

(eval-after-load 'projectile
  '(add-hook 'find-file-hook 'camdez/warn-if-longhorn-version))

(defun camdez/warn-if-longhorn-version ()
  (interactive)
  (when (projectile-project-p)
    (let* ((file-name    (buffer-file-name))
           (project-dir  (projectile-project-root))
           (views-dir    (concat project-dir "app/views/"))
           (lh-views-dir (concat project-dir "app/longhorn_views/")))
      (when (string-match (concat "^" views-dir) file-name)
        (let* ((file-rel-name (substring file-name (match-end 0)))
               (lh-file-name  (concat lh-views-dir file-rel-name)))
          (when (file-exists-p lh-file-name)
            (message "Warning: there is a Longhorn version of this file")))))))

For @amirrajan. :)