Today, I've tried to solve the problem of why my LaTeX-installation (MikTex) produces pdf-files instead of dvi-files with some documents, even though I have set the output profile to LaTeX => DVI in TexnicCenter. Since I have collected some references which where not that easy to find, I just post them here:

The clearest answer seems to be this one:
http://lists.debian.org/debian-tetex-maint/2006/01/msg00409.html"
and similarly this one:
http://www.issa.stevens.edu/cgi-bin/query-pr-full?pr=2937

yes, it is as these people write: in the tex-file in question I do find this:
\newif\ifpdf
\ifx\pdfoutput\undefined
\pdffalse
\else
\pdfoutput=1
\pdftrue
\fi

(It is in fact from a template by Mark Müller, based on KOMA-script. I think it came with TexnicCenter)

I have replaced the first two line with

\usepackage{ifpdf}
\ifpdf

But a pdf-environment is still detected (and thus pdf instead of dvi produced), I don't know why... :-((

Here are some more documents I googled:
The description of the \ifpdf package:
ftp://tug.ctan.org/pub/tex-archive/macros/latex/contrib/oberdiek/ifpdf.pdf

and a manual which still uses the old pdf-detection, which - according to the sources above - does not work anymore:
http://tex.lickert.net/tipps/ausgabetest/format_abfragen.html