If you want to take advantage of the powerful \newtheorem command
without the constraint that the contents of the theorem is in a sloped
font (for example, to use it to create remarks, examples, proofs,
...) then you can use the style file theorem.sty (part of
macros/latex/packages/tools). Alternatively, the following sets up an
environment remark whose content is in roman.
\newtheorem{preremark}{Remark}
\newenvironment{remark}%
{\begin{preremark}\rm}{\end{preremark}}
This will not work if you are using
NFSS outside of LaTeX2e,
because the command \rm behaves differently there.