Emacs 23 has this integrated in whitespace-mode. Add lines and lines-tail to whitespace-style, and customize whitespace-line-column
This mode highlights all characters that cross the 80 character line limit.
Add the following to your .emacs:
(add-to-list 'load-path "/path/to/highlight-80+") (require 'highlight-80+)
Enable the mode with M-x highlight-80+-mode
If you have any feedback, please email me, or visit this package's page at the Emacs wiki.