Theorem environment - line break after label

this is a "break" style recommended in the ams' "newtheorem and theoremstyle test" for amsthm:

\newtheoremstyle{break}% name
  {}%         Space above, empty = `usual value'
  {}%         Space below
  {\itshape}% Body font
  {}%         Indent amount (empty = no indent, \parindent = para indent)
  {\bfseries}% Thm head font
  {.}%        Punctuation after thm head
  {\newline}% Space after thm head: \newline = linebreak
  {}%         Thm head spec

\theoremstyle{break}
\newtheorem{thm}{Theorem}

look here for the pdf output and the input for this documentation. the files (named thmtest.*) are also on ctan.

The answer depends on the package used to declare the theorem-like structures. If you are using the ntheorem package, you can simply use the predefined break style:

\documentclass{article}
\usepackage{ntheorem}
\usepackage{lipsum}

\theoremstyle{break}
\newtheorem{theorem}{Theorem}

\begin{document}

\begin{theorem}[Some note]
\lipsum*[2]
\end{theorem}

\end{document}

If you are using amsthm, you will have to define your own style through the \newtheoremstyle command. Here's an example of such a definition:

\documentclass{article}
\usepackage{amsthm}
\usepackage{lipsum}

\newtheoremstyle{break}
  {\topsep}{\topsep}%
  {\itshape}{}%
  {\bfseries}{}%
  {\newline}{}%
\theoremstyle{break}
\newtheorem{theorem}{Theorem}

\begin{document}

\begin{theorem}[Some note]
\lipsum*[2]
\end{theorem}

\end{document}