%% 
%%  A DANTE-Edition example
%% 
%%  Beispiel 05-01-4 auf Seite 169.
%% 
%%  Copyright (C) 2010 H. Voss
%% 
%%  It may be distributed and/or modified under the conditions
%%  of the LaTeX Project Public License, either version 1.3
%%  of this license or (at your option) any later version.
%% 
%%  See http://www.latex-project.org/lppl.txt for details.
%% 

%%Run also: >>  << 
% Show page(s) 1

\documentclass[]{article}
\pagestyle{empty}
\setlength\textwidth{146.59192pt}
\usepackage[utf8]{inputenc}
\usepackage[ngerman]{babel}
\setlength\parindent{0pt}
\usepackage{longtable}
\makeatletter
\newcommand*{\org@longtable}{}
\let\org@longtable\longtable
\def\longtable{%
  \begingroup
    \advance\c@LT@tables\@ne
    \edef\x{LT@\romannumeral\c@LT@tables}%
    \global\LongtableWidth\z@
    \@ifundefined{\x}{% longtable width not available
    }{\def\LT@entry##1##2{%
      \global\advance\LongtableWidth##2\relax}%
      \@nameuse{\x}}%
    \typeout{* \x: \the\LongtableWidth}%debug output
  \endgroup
  \ifdim\LongtableWidth>\z@
    \setlength{\LTcapwidth}{\LongtableWidth}%
  \fi
  \org@longtable}
\makeatother

\usepackage{longtable}
\newlength\LongtableWidth% Siehe Beispielcode

\begin{document}
\begin{longtable}{|ll|}
\caption{Caption für eine \texttt{longtable}\dotfill}\\
Hallo & Welt und etwas mehr\\ foo & bar
\end{longtable}
\begin{center}
\begin{minipage}{\LongtableWidth}
Hier kommt jetzt Text in der Breite
der \texttt{longtable}
\end{minipage}
\end{center}
\end{document}