Since everyone is telling you not to do it, here;s one way of doing it:-)

\documentclass{beamer}
\let\oldframe\frame
\renewcommand\frame[1][allowframebreaks]{\oldframe[#1]}
\begin{document}
\begin{frame}
\begin{enumerate}
\item something \item something \item something
\item something \item something \item something
\item something \item something \item something
\item something \item something \item something
\item something \item something \item something
\item something \item something \item something
\item something \item something \item something
\item something \item something \item something
\item something \item something \item something
\item something \item something \item something
\end{enumerate}
\end{frame}
\end{document}
That leaves breaking as the default option but if you specify any other option the default
is not used so you need to include allowframebreaks whenever you have an option.
If you definitely always want it (rather than just having it as default if no option used) you can use instead of the above
\renewcommand\frame[1][]{\oldframe[allowframebreaks,#1]}
so that allowframebreaks is always prepended to the option list.
\beamer@@@@frame. This can probably be removed usingetoolbox, and the key then fixed as true using\setkeys{beamerframe}{allowframebreaks}. However, this is really against the whole concept of howbeameris structured. – Joseph Wright Mar 01 '13 at 20:15allowframebreaks, BUT: It makes my life easier and (as the programmer part of me says "keep the code DRY"). Don't have to make frames with identical headings as I can use manual\framebreakinstead. – mreq Mar 01 '13 at 20:17