I use the following function from the "find-file-hook" in order to choose between using objc-mode, c++-mode and c-mode when opening a ".h" file:

(defun bh-choose-header-mode ()
  (interactive)
  (if (string-equal (substring (buffer-file-name) -2) ".h")
      (progn
        ;; OK, we got a .h file, if a .m file exists we'll assume it's
        ; an objective c file. Otherwise, we'll look for a .cpp file.
        (let ((dot-m-file (concat (substring (buffer-file-name) 0 -1) "m"))
              (dot-cpp-file (concat (substring (buffer-file-name) 0 -1) "cpp")))
          (if (file-exists-p dot-m-file)
              (progn
                (objc-mode)
                )
            (if (file-exists-p dot-cpp-file)
                (c++-mode)
              )
            )
          )
        )
    )
  )

(add-hook 'find-file-hook 'bh-choose-header-mode)