\documentclass{article}

\usepackage{type1cm}
\usepackage{lettrine}

\makeatletter
\newenvironment{thm}{%
  \trivlist
  \@topsep=\topsep
  \@topsepadd=\topsep
  \item                             %% <- it doesn't need a label
  \def\LettrineFontHook{\normalfont\bfseries}%
  \def\LettrineTextFont{\normalfont\bfseries}%
  \lettrine[nindent=0em, findent=0.125em, realheight]{T}{heorem.}%
  \everypar{\parshape0\everypar{}}% %% <- reset \parshape
  \itshape\space\ignorespaces
}{%
  \endtrivlist
  \@endpefalse
}
\makeatother

\def\lipsum{Lorem ipsum dolor sit amet, consectetuer adipiscing elit.
           Ut purus elit, vestibulum ut, placerat ac, adipiscing vitae, felis.
           Curabitur dictum gravida mauris. Nam arcu libero, nonummy eget,
           consectetuer id, vulputate a, magna. Donec vehicula augue eu neque.}

\begin{document}

\lipsum

\begin{thm}
    \lipsum

    \lipsum
\end{thm}

\lipsum

\end{document}