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\documentclass{beamer}
\begin{document}
\frame
{\begin{itemize}
\item<1-> Punkt 1
\item<2-> Punkt 2
\item<3-> Punkt 3
\end{itemize}
\only<4->{
\begin{tabular}{cc}
\hline
Zeile 1 & Zeile 1\\
\hline
\only<5->{Zeile 2 & Zeile 2\\
\hline}
\end{tabular}
}
}
\end{document}

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\documentclass{beamer}
\begin{document}
\frame
{\only<1->{
\begin{tabular}{cc}
\hline
\only<1->{Zeile 1} & \only<3->{Zeile 1}\\
\hline
\only<2->{Zeile 2} & \only<4->{Zeile 2}\\
\hline
\end{tabular}
}
}
\end{document}

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\documentclass{beamer}
\newlength{\bla}
\settowidth{\bla}{Text der breitesten Zelle}
\begin{document}
\frame
{\only<1->{
\begin{tabular}{cc}
\hline
\only<1->{Zeile 1} & \only<1-2>{\hspace*{\bla}}\only<3->{Text der breitesten Zelle}\\
\hline
\only<2->{Zeile 2} & \only<4->{Zeile 2}\\
\hline
\end{tabular}
}
}
\end{document}