You can set the headline beamer template to be empty locally, using \setbeamertemplate{headline}{}; if the frame has space reserved for a frame title, in addition to the modification mentioned before, you will have to move the frame title upwards for this frame, and this can be done by adding a convenient \vspace{<length>} to the frametitle template (also locally). A little example removing the navigation bar from the frame containing \maketitle and for the frames containing the ToCs generated with \AtBeginSection:
\documentclass{beamer}
\usetheme{Copenhagen}
\AtBeginSection[] % Do nothing for \section*
{
\begingroup
\setbeamertemplate{headline}{}
\addtobeamertemplate{frametitle}{\vspace*{-0.9\baselineskip}}{}
\begin{frame}<beamer>
\frametitle{Summary}
\tableofcontents[currentsection]
\end{frame}
\endgroup
}
\title{The Title}
\author{The Author}
\institute{The Institute}
\begin{document}
{
\setbeamertemplate{headline}{}
\begin{frame}
\maketitle
\end{frame}
}
\section{Test Section One}
\begin{frame}{The frame title}
test one
\end{frame}
\begin{frame}{The frame title}
test two
\end{frame}
\section{Test Section Two}
\begin{frame}{The frame title}
test three
\end{frame}
\begin{frame}{The frame title}
test four
\end{frame}
\end{document}
The resulting document:

For an specific frame the same principle applies:
{
\setbeamertemplate{headline}{}
\addtobeamertemplate{frametitle}{\vspace*{-0.9\baselineskip}}{}
\begin{frame}
\frametitle{Summary}
Some text here
\end{frame}
}
A complete example, as required in a comment;
\documentclass{beamer}
\usetheme{Copenhagen}
\title{The Title}
\author{The Author}
\institute{The Institute}
\begin{document}
{
\setbeamertemplate{headline}{}
\begin{frame}
\maketitle
\end{frame}
}
{
\setbeamertemplate{headline}{}
\addtobeamertemplate{frametitle}{\vspace*{-0.9\baselineskip}}{}
\begin{frame}
\frametitle{Summary}
Some text here
\end{frame}
}
\section{Test Section One}
\begin{frame}{The frame title}
test one
\end{frame}
\begin{frame}{The frame title}
test two
\end{frame}
\section{Test Section Two}
\begin{frame}{The frame title}
test three
\end{frame}
\begin{frame}{The frame title}
test four
\end{frame}
\end{document}
plainoption to bothframes. – Paul Gaborit Oct 10 '12 at 20:59plainoption is the simpler solution. – Paul Gaborit Oct 10 '12 at 21:33