I think that all theorem-like environments should be treated in the same way, so a solution can be to use list instead of trivlist in the definitions, thus making it possible to use \leftmargin:
\usepackage{etoolbox}
\makeatletter
\patchcmd{\@thm}{\trivlist}{\list{}{\leftmargin=2.5em}}{}{}
\patchcmd{\@endtheorem}{\endtrivlist}{\endlist}{}{}
\makeatother
Now statements such as
\newtheoremstyle{indentedupright}
{3pt}
{3pt}
{}
{}
{\bfseries}
{.}
{.5em}
{}
for definitions and
\newtheoremstyle{indenteditalic}
{3pt}
{3pt}
{\itshape}
{}
{\bfseries}
{.}
{.5em}
{}
for theorems will do.
Here's an extended version that allows to have different indentations for the various theorem-like environments:
\documentclass{amsbook}
\usepackage{amsmath,amssymb,amsthm}
\usepackage[english]{babel}
\usepackage{blindtext}
\usepackage{etoolbox}
\makeatletter
\patchcmd{\@thm}{\trivlist}{\list{}{\leftmargin=\thm@margin}}{}{}
\patchcmd{\@endtheorem}{\endtrivlist}{\endlist}{}{}
\newlength{\thm@margin}
\newcommand{\xnewtheorem}[2][0pt]{%
\newenvironment{#2}{\thm@margin=#1 \begin{#2INNER}}{\end{#2INNER}}%
\newtheorem{#2INNER}%
}
\makeatother
\newtheoremstyle{indentedupright}
{3pt}
{3pt}
{}
{}
{\bfseries}
{.}
{.5em}
{}
\newtheoremstyle{indenteditalic}
{3pt}
{3pt}
{\itshape}
{}
{\bfseries}
{.}
{.5em}
{}
\theoremstyle{indenteditalic}
\xnewtheorem{theorem}{Theorem}
\theoremstyle{indentedupright}
\xnewtheorem[2.5em]{definition}{Definition}
\begin{document}
\blindtext
\begin{definition}[Companion and cut-off event]
\label{def:cutoff}
Let $\beta$ be a branching process and let $\prec$ be an adequate partial order on the
configurations of $\beta$. An event $e$ is a \emph{cut-off event} (with respect to $\prec$)
if $\beta$ contains a local configuration $[e']$ such that
\begin{enumerate}
\item $Mark(e) = Mark(e')$, and \label{def:cutoff-markequal}
\item $[e'] \prec [e]$ \label{def:cutoff-smaller}
\end{enumerate}
Another way to express the condition \ref{def:cutoff-markequal} is $e' \in [e]_{Mark}$. The
event $e'$ is called \emph{companion} of $e$, if in addition it is minimal in $[e]_{Mark}$
with respect to $\prec$. Moreover, if $\prec$ is total, then the companion of a cut-off
event $e$ is uniquely defined and is denoted by $e_{\mathfrak{c}}$.
\end{definition}
\blindtext
\begin{theorem}
This is the statement of an important theorem; however we don't want to have an indentation
for this kind of statement.
\end{theorem}
\end{document}
Note that if a new theorem must share a counter with another, the syntax should be like
\xnewtheorem{prop}[theoremINNER]{Proposition}
(with or without the optional argument for the indentation).
A variant for specifying both the left and the right margin; in this case the lengths are mandatory arguments; it would be possible to add a key-value interface instead.
\documentclass{amsbook}
\usepackage{amsmath,amssymb,amsthm}
\usepackage[english]{babel}
\usepackage{etoolbox}
\usepackage{lipsum}
\makeatletter
\patchcmd{\@thm}
{\trivlist}
{\list{}{\leftmargin=\thm@leftmargin\rightmargin=\thm@rightmargin}}
{}{}
\patchcmd{\@endtheorem}
{\endtrivlist}
{\endlist}
{}{}
\newlength{\thm@leftmargin}
\newlength{\thm@rightmargin}
\newcommand{\xnewtheorem}[3]{%
\newenvironment{#3}
{\thm@leftmargin=#1\relax\thm@rightmargin=#2\relax\begin{#3INNER}}
{\end{#3INNER}}%
\newtheorem{#3INNER}%
}
\makeatother
\newtheoremstyle{indentedupright}
{3pt}
{3pt}
{}
{}
{\bfseries}
{.}
{.5em}
{}
\newtheoremstyle{indenteditalic}
{3pt}
{3pt}
{\itshape}
{}
{\bfseries}
{.}
{.5em}
{}
\theoremstyle{indenteditalic}
\xnewtheorem{0pt}{0pt}{theorem}{Theorem}
\theoremstyle{indentedupright}
\xnewtheorem{2.5em}{0pt}{definition}{Definition}
\xnewtheorem{2.5em}{2.5em}{example}{Example}
\begin{document}
\lipsum[2]
\begin{definition}[Companion and cut-off event]\label{def:cutoff}
Let $\beta$ be a branching process and let $\prec$ be an adequate partial order on the
configurations of $\beta$. An event $e$ is a \emph{cut-off event} (with respect to $\prec$)
if $\beta$ contains a local configuration $[e']$ such that
\begin{enumerate}
\item $Mark(e) = Mark(e')$, and \label{def:cutoff-markequal}
\item $[e'] \prec [e]$ \label{def:cutoff-smaller}
\end{enumerate}
Another way to express the condition \ref{def:cutoff-markequal} is $e' \in [e]_{Mark}$. The
event $e'$ is called \emph{companion} of $e$, if in addition it is minimal in $[e]_{Mark}$
with respect to $\prec$. Moreover, if $\prec$ is total, then the companion of a cut-off
event $e$ is uniquely defined and is denoted by $e_{\mathfrak{c}}$.
\end{definition}
\lipsum[3]
\begin{theorem}
This is the statement of an important theorem; however we don't want to have an indentation
for this kind of statement.
\end{theorem}
\lipsum[3]
\begin{example}
\lipsum[4]
\end{example}
\end{document}
