### Roman theorems

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 AMSLaTeX amsthm package (which now supersedes the theorem package previously recommended in these answers). Alternatively, the following sets up an environment `remark` whose content is in roman.

      \newtheorem{preremark}{Remark}
      \newenvironment{remark}%
        {\begin{preremark}\upshape}{\end{preremark}}

The ntheorem package provides roman theorems directly.

`amsthm.sty`
  Distributed as part of macros/latex/required/amslatex (gzipped tar, browse)
`ntheorem.sty`
  macros/latex/contrib/ntheorem (gzipped tar, browse)
`theorem.sty`
  Distributed as part of macros/latex/required/tools (gzipped tar, browse)

This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=theoremfmt