Justify text inside itemize environment with beamer class

beamer has its own ways and, in particular, it redefines many standard LaTeX environments/commands; in particular, it redefines the basic list-like environments itemize, enumerate and description.

Here's, for example, the definition of \itemize as given in beamerbaselocalstructure.sty:

\renewcommand{\itemize}[1][]{%
  \beamer@ifempty{#1}{}{\def\beamer@defaultospec{#1}}%
  \ifnum \@itemdepth >2\relax\@toodeep\else
    \advance\@itemdepth\@ne
    \beamer@computepref\@itemdepth% sets \beameritemnestingprefix
    \usebeamerfont{itemize/enumerate \beameritemnestingprefix body}%
    \usebeamercolor[fg]{itemize/enumerate \beameritemnestingprefix body}%
    \usebeamertemplate{itemize/enumerate \beameritemnestingprefix body begin}%
    \list
      {\usebeamertemplate{itemize \beameritemnestingprefix item}}
      {\def\makelabel##1{%
          {%
            \hss\llap{{%
                \usebeamerfont*{itemize \beameritemnestingprefix item}%
                \usebeamercolor[fg]{itemize \beameritemnestingprefix item}##1}}%
          }%
        }%
      }
  \fi%
  \beamer@cramped%
  \raggedright% 
  \beamer@firstlineitemizeunskip%
}

as you can see, \itemize internally uses \raggedright. The best approach to keep things overlay specification-aware is to redefine \itemize to use \justifying (from the ragged2e package) instead; the following code shows this redefinition in an actual document:

\documentclass{beamer}
\usepackage{ragged2e}
\usepackage{lipsum}

\makeatletter
\renewcommand{\itemize}[1][]{%
  \beamer@ifempty{#1}{}{\def\beamer@defaultospec{#1}}%
  \ifnum \@itemdepth >2\relax\@toodeep\else
    \advance\@itemdepth\@ne
    \beamer@computepref\@itemdepth% sets \beameritemnestingprefix
    \usebeamerfont{itemize/enumerate \beameritemnestingprefix body}%
    \usebeamercolor[fg]{itemize/enumerate \beameritemnestingprefix body}%
    \usebeamertemplate{itemize/enumerate \beameritemnestingprefix body begin}%
    \list
      {\usebeamertemplate{itemize \beameritemnestingprefix item}}
      {\def\makelabel##1{%
          {%
            \hss\llap{{%
                \usebeamerfont*{itemize \beameritemnestingprefix item}%
                \usebeamercolor[fg]{itemize \beameritemnestingprefix item}##1}}%
          }%
        }%
      }
  \fi%
  \beamer@cramped%
  \justifying% NEW
  %\raggedright% ORIGINAL
  \beamer@firstlineitemizeunskip%
}
\makeatother

\begin{document}

\begin{frame}
\begin{itemize}
\item<1->\lipsum[2]
\item<2>\lipsum[2]
\end{itemize}
\end{frame}

\end{document}

The output:

As a final note, I would suggest you to consider if this redefinition is really needed at all; frames with too much text (like the one I just build for the example) are to be avoided in a presentation.

Try this.

\documentclass{beamer}
\usepackage{ragged2e}
\usepackage{lipsum}
\let\olditem\item
\renewcommand\item{\olditem\justifying}
\begin{document}
\begin{frame}
\begin{itemize}
\item \lipsum[1-1]
\end{itemize}
\end{frame}
\end{document}

use the package ragged2e

\begin{itemize}
        \justifying
    \item...
    \item...
\end{itemize}