I would try:
(font-lock-add-keywords nil '("^.*//!.*" 0 font-lock-warning-face prepend))
--
__Pascal Bourguignon__ http://www.informatimago.com/
Hi Pascal thanks so much that is perfect :) For any interested users
that come along later, I ended up specifically using:
(add-hook 'c++-mode-hook (lambda () (font-lock-add-keywords nil
'(("//!.*" 0 font-lock-warning-face t)))))
because highlighting the whole line turned out to be very distracting.