1. book class
Method 1 — Define a new command
The \ftableofcontents command wants as argument the text of the footnote.
\documentclass{book}
\usepackage{xpatch}
\newcommand\ftableofcontents{\renewcommand\ftableofcontents[1]}
\expandafter\ftableofcontents\expandafter{\tableofcontents}
\xpatchcmd\ftableofcontents
{\contentsname}
{\contentsname\footnote{#1}\fi}{}{}
% Omit the following lines if you want the footnote with an arabic number
\xpretocmd\ftableofcontents{\begingroup\renewcommand\thefootnote{\fnsymbol{footnote}}}{}{}
\xapptocmd\ftableofcontents{\endgroup}{}{}
\begin{document}
\frontmatter
\ftableofcontents{This is the footnote}
\mainmatter
\chapter{A chapter}
\section{A section}
Here is a footnote\footnote{A}
\end{document}
Method 2 — Overload \tableofcontents
In this case we redefine \tableofcontents to accept as optional argument the text of the footnote; nothing happens if no optional argument is specified.
\documentclass{book}
\usepackage{xpatch}
\newcommand\temp{\renewcommand\tableofcontents[1][]}
\expandafter\temp\expandafter{\tableofcontents}
\xpatchcmd\tableofcontents
{\contentsname}
{\contentsname\if\relax\detokenize{#1}\relax\else\footnote{#1}\fi}{}{}
% Omit the following lines if you want the footnote with an arabic number
\xpretocmd\tableofcontents{\begingroup\renewcommand\thefootnote{\fnsymbol{footnote}}}{}{}
\xapptocmd\tableofcontents{\endgroup}{}{}
\begin{document}
\frontmatter
\tableofcontents[This is the footnote]
\mainmatter
\chapter{A chapter}
\section{A section}
Here is a footnote\footnote{A}
\end{document}

2. scrbook class
The previous methods don't work with the scrbook class and it would be very difficult to patch the commands that produce the table of contents, so a hack seems the best thing to try (apart avoiding putting a footnote to a chapter title to begin with).
\documentclass{scrbook}
\begin{document}
\frontmatter
\begingroup
\makeatletter
\protected@edef\contentsname{\contentsname\unexpanded{\footnote{%
%%%% ADD HERE THE FOOTNOTE TEXT
This is a footnote%
%%%% DON'T FORGET THE FINAL %
}}%
\gdef\noexpand\contentsname{\contentsname}}
% Omit the following line if you want a number
\renewcommand\thefootnote{\fnsymbol{footnote}}
\tableofcontents
\endgroup
\mainmatter
3. Alternative method (all classes)
Instead of a footnote you can add some text just after the header.
\documentclass{scrbook}
\begin{document}
\frontmatter
\addtocontents{toc}{\unexpanded{\unexpanded{{%
\itshape\footnotesize
This is a note for the table of contents
\par\bigskip
}}}}
\tableofcontents
\mainmatter
\chapter{A chapter}
\section{A section}
\end{document}
