\setbeamertemplate{block begin}{
\vskip.75ex
\begin{beamercolorbox}[rounded=true,shadow=true,leftskip=1cm,colsep*=.75ex]{block
title}%
\usebeamerfont*{block title}\insertblocktitle
\end{beamercolorbox}%
{\ifbeamercolorempty[bg]{block body}{}{\nointerlineskip\vskip-0.5pt}}%
\usebeamerfont{block body}%
\begin{beamercolorbox}[rounded=true,shadow=true,colsep*=.75ex,sep=.75ex,vmode]{block
body}%
\ifbeamercolorempty[bg]{block body}{\vskip-.25ex}{\vskip-.75ex}\vbox{}%
}
\setbeamertemplate{block end}{
\end{beamercolorbox}
}
See also some examples at
http://www-i6.informatik.rwth-aachen.de/~dreuw/latexbeamerposter.php