Several people have asked me how I make TODO appear in the margin of my thesis wherever I want to highlight an area that needs further work. The trick is to define a new LaTeX command called “todo” in your pre-amble:
\newcommand{\todo}[1]{\marginpar{\textsf{\textbf{TODO}}}
\texttt{\small{TODO:#1}}}
(all on one line)

TODO items can then be marked up in the text as:
\todo{Add a reference here.}