\documentclass{article}
\usepackage{xparse}
\NewDocumentCommand\mylist{>{\SplitList{;}}m}
{
\begin{itemize}
\ProcessList {#1} { \insertitem}
\end{itemize}
}
\newcommand\insertitem[1]{\item #1}
\begin{document}
Short list:
\mylist{a;b}
Longer list:
\mylist{a;b;c;d}
List within a list:
\mylist{a;b\mylist{A;B;C;D};c;d}
\end{document}