As inspired by the comic WTFs per minute, we'd thought that besides statements like TODO and FIXME, we would 'really' like vi to highlight the string WTF.
First, you need to locate the syntax file:
$ locate syntax/c.vim /usr/share/vim/vim70/syntax/c.vim
If you're root, just edit this file, search for the line:
syn keyword cTodo contained TODO FIXME XXX
and add WTF at the end of the line.
If you're a mere mortal user, copy this file to your .vimrc directory in your homedir and give it a name such as my_c.vim. Then edit the file as described above and add the following lines to your $HOME/.vimrc file:
au BufRead,BufNewFile *.c set filetype=myc au! Syntax myc source $HOME/myc.vim