Continuing enumerate Counters in Beamer

Use \resetcounteronoverlays to ensure that your counter is automatically reset on subsequent slides of a frame:

\resetcounteronoverlays{saveenumi}

A complete example using your code:

\documentclass{beamer}
\setbeamercovered{highly dynamic}

\newcounter{saveenumi}
\newcommand{\seti}{\setcounter{saveenumi}{\value{enumi}}}
\newcommand{\conti}{\setcounter{enumi}{\value{saveenumi}}}

\resetcounteronoverlays{saveenumi}

\begin{document}

\begin{frame}[<+->]
  \begin{enumerate}
  \item foo
  \item bar%
    \seti
  \end{enumerate}
\end{frame}

\begin{frame}[<+->]
  \begin{enumerate}
    \conti
  \item zip
  \item yadda%
    \seti
  \end{enumerate}
\end{frame}

\end{document}

Try resetting the enumi counter in the new list, like \setcounter{enumi}{5} to initiate with 6.