\documentclass[a4paper,11pt]{article}

\usepackage[hebrew,english]{babel}
\usepackage{amsthm}

\newtheorem{theorem}{Theorem}
\begin{document}
\begin{theorem}
Blah.
\end{theorem}
\end{document}

% TeX'ing this with MikTeX 2.5 yields :
% ! Missing { inserted.
% <to be read again>
%                    \special
% l.9 B
%      lah.