Linking the section text to the TOC

Here's another approach, using the titlesec package and the \hypertarget, \hyperlink mechanism from hyperref. A little example showing the necessary settings for \chapter, \section, \subsection and their starred versions:

\documentclass{book}
\usepackage[explicit]{titlesec}
\usepackage{hyperref}

\titleformat{\chapter}[display]
  {\normalfont\huge\bfseries}{\chaptertitlename\ {\fontfamily{cmr}\selectfont\thechapter}}{20pt}{\hyperlink{chap-\thechapter}{\Huge#1}
\addtocontents{toc}{\protect\hypertarget{chap-\thechapter}{}}}
\titleformat{name=\chapter,numberless}
  {\normalfont\huge\bfseries}{}{-20pt}{\Huge#1}
\titleformat{\section}
  {\normalfont\Large\bfseries}{\thesection}{1em}{\hyperlink{sec-\thesection}{#1}
\addtocontents{toc}{\protect\hypertarget{sec-\thesection}{}}}
\titleformat{name=\section,numberless}
  {\normalfont\Large\bfseries}{}{0pt}{#1}
\titleformat{\subsection}
  {\normalfont\large\bfseries}{\thesubsection}{1em}{\hyperlink{subsec-\thesubsection}{#1}
\addtocontents{toc}{\protect\hypertarget{subsec-\thesubsection}{}}}
\titleformat{name=\subsection,numberless}
  {\normalfont\large\bfseries}{\thesubsection}{0pt}{#1}

\begin{document}

\tableofcontents

\chapter{Numbered test chapter one}

\section{Test section one}

\subsection{test subsection one one}

\subsection{test subsection one two}

\chapter*{Unnumbered test chapter one}

\section*{Unnumbered test section}

\subsection*{Unnumbered test subsection}

\end{document}

This solution sets up \hypertargets in the toc by renewing the \contentsline command. It links back to the toc by setting the section headings as \hyperlinks using the titlesec package. Note that each of the new \hypertarget start with the word 'toc'

You can easily copy this approach for chapters etc.

\documentclass{article}
\usepackage{lipsum} % sample text
\usepackage[explicit]{titlesec} % to change headings
\usepackage{hyperref}

% renew \contentsline for toc to include hypertarget
\let\oldcontentsline\contentsline%
\renewcommand\contentsline[4]{%
\hypertarget{toc#4}{}%
\oldcontentsline{#1}{#2}{#3}{#4}}

% renew \section to link to the toc
\titleformat{\section}
{\normalfont\Large\bf}
{{\thesection} \hyperlink{tocsection.\thesection}{#1}}
{1pc}
{}

% renew \subsection to link to the toc
\titleformat{\subsection}
{\normalfont\bf}
{{\thesection} \hyperlink{tocsubsection.\thesubsection}{#1}}
{1pc}
{}

\begin{document}
\tableofcontents

\clearpage

\section{First section}
\lipsum[1]
\subsection{my sub section}
\lipsum[1]

\section{Second section}
\lipsum[2]

\clearpage

\section{Third section}
\lipsum[3]
\end{document}

Explanation

Note that if you look at the .toc file for the above example you'll see the following entries

\contentsline {section}{\numberline {1}First section}{2}{section.1}
\contentsline {subsection}{\numberline {1.1}my sub section}{2}{subsection.1.1}
\contentsline {section}{\numberline {2}Second section}{2}{section.2}
\contentsline {section}{\numberline {3}Third section}{3}{section.3}

The \contentsline takes four arguments, the fourth of which will uniquely identify each entry. I used this unique entry combined with the string 'toc' as the \hypertarget{toc#4}{}; it would not work without the additional string 'toc' (or something similar), as it would result in a duplicated identifier which would be ignored.

I wrote a very simple example of myself.

\documentclass{article}
\usepackage{lipsum}
\usepackage{hyperref}

\let\oldsection\section
\renewcommand\section[1]{%
    \addtocontents{toc}{\protect\hypertarget{#1}{}}
    \oldsection{\texorpdfstring{\protect\hyperlink{#1}{#1}}{#1}}
    } %%

\let\oldsubsection\subsection
\renewcommand\subsection[1]{%
    \addtocontents{toc}{\protect\hypertarget{#1}{}}
    \oldsubsection{\texorpdfstring{\protect\hyperlink{#1}{#1}}{#1}}
    } %%

\begin{document}
{
    \let\section\oldsection
    \pdfbookmark{Contents}{Contents} 
    \vspace*{.5cm}
    \tableofcontents
}
\clearpage

\section{First section}
\lipsum[1]
\subsection{my sub section}
\lipsum[1]

\section{Second section}
\lipsum[2]

\clearpage

\section{Third section}
\lipsum[3]
\end{document}