Brace and text in side margin

You can use a \makebox to define a new command with two arguments: the first one will contain the text for the brace, and the second one will contain the paragraph text; something along these lines:

\documentclass{report} 
\usepackage{amsmath}
\usepackage{lipsum}

\newcommand\BrText[2]{%
  \par\smallskip
   \noindent\makebox[\textwidth][r]{$\text{#1}\left\{
    \begin{minipage}{\textwidth}
    #2
    \end{minipage}
  \right.\nulldelimiterspace=0pt$}\par\smallskip
}    

\begin{document}
\lipsum[1]
\BrText{Some text}{\lipsum[1]}
\lipsum[1]
\end{document}

Here is another approach that utilized the environ package. The two-step procedure boxes the contents (in \savetextbox) in order to get its height. Then it typesets the left brace in the margin, and then the box.

In the following minimal working example, a new environment bracetext is defined. It takes an optional argument which specifies the width of the text block (default is \textwidth). It is possible to modify this environment to take arguments (optional or mandatory) for the text to be displayed on the left-hand side as well.

\documentclass{report}
\usepackage{environ}% http://ctan.org/pkg/environ
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\newbox{\savetextbox}
\NewEnviron{bracetext}[1][\textwidth]{%
  \begin{lrbox}{\savetextbox}%
    \begin{minipage}{#1} \BODY \end{minipage}
  \end{lrbox}%
  \smallskip%
  \noindent\makebox[0pt][r]{$\left\{\rule{0pt}{\ht\savetextbox}\right.$}%
  \usebox{\savetextbox}\par
  \smallskip%
}
\begin{document}
  \lipsum[1]
  \begin{bracetext}
    \lipsum[2]
  \end{bracetext}
  \lipsum[3]
\end{document}​

The lipsum package was used to provide dummy text Lorem Ipsum... style.

Note this one caveat of this approach is that the brace will not split across pages. If you want something that splits across pages, you should consider using mdframed. However, it does not support bracing as you requested.