[pstricks] handling of units

[Goebel, Juergen, ISO31]

Hi,

there is a problem for me handling units in pstricks. Please have 
a look at the following example.

\documentclass{article}
\usepackage{pstricks}
\begin{document}
 \psset{unit=1mm}
 \begin{pspicture}(0,0)(100,100)
  \newlength{\mylength}
  \setlength{\mylength}{10mm}
  \psframe(0,0)(10,10)
  \makeatletter
  \psframe(0,0)(\strip at pt\mylength,\strip at pt\mylength)
  \makeatother
 \end{pspicture}
\end{document}

After using \strip at pt I get a frame, that is approx 2.83 times
larger then I'd like it to be. So pstricks uses big points here, 
although the unit is set explicitly to 1mm. What do I have to do?

TIA

Jürgen