Nested itemize environment affects vertical spacing of parent itemize environment
The package enumitem
somehow solves the problem. Note that the package redefines beamer list features.
\documentclass{beamer}
\usepackage{enumitem}
% redefine default beamer item labels
\setitemize{label=\usebeamerfont*{itemize item}%
\usebeamercolor[fg]{itemize item}
\usebeamertemplate{itemize item}}
\begin{document}
\begin{frame}
\begin{itemize}
\item a
\item b
\item c
\begin{itemize}
\item d
\item e
\end{itemize}
\end{itemize}
\end{frame}
\end{document}
This issue appears to be solved by leaving a blank line before the nested itemize:
\documentclass{beamer}
\begin{document}
\begin{frame}
\begin{itemize}
\item a
\item b
\item c
\begin{itemize}
\item d
\item e
\end{itemize}
\end{itemize}
\end{frame}
\end{document}