http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=363371
Somebody (you?) opened a bug report for this one.
(which doesn't give you the solution..)
edit:
on my sid it works, if it can help:
me@debian:~/Desktop$ xlsfonts | egrep "^-.*-helvetica-medium-r-normal--1.*-iso8859-1$"
-adobe-helvetica-medium-r-normal--11-80-100-100-p-56-iso8859-1
-adobe-helvetica-medium-r-normal--11-80-100-100-p-56-iso8859-1
-adobe-helvetica-medium-r-normal--11-80-100-100-p-56-iso8859-1
-adobe-helvetica-medium-r-normal--14-100-100-100-p-76-iso8859-1
-adobe-helvetica-medium-r-normal--14-100-100-100-p-76-iso8859-1
-adobe-helvetica-medium-r-normal--14-100-100-100-p-76-iso8859-1
-adobe-helvetica-medium-r-normal--17-120-100-100-p-88-iso8859-1
-adobe-helvetica-medium-r-normal--17-120-100-100-p-88-iso8859-1
-adobe-helvetica-medium-r-normal--17-120-100-100-p-88-iso8859-1
debian:~# dpkg -l defoma imagemagick xfs xfonts-100dpi libfreetype6 libx11-6
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Installed/Config-files/Unpacked/Failed-config/Half-installed
|/ Err?=(none)/Hold/Reinst-required/X=both-problems (Status,Err: uppercase=bad)
||/ Name Version Description
+++-======================-======================-=================================
ii defoma 0.11.8-0.1 Debian Font Manager -- automatic font configuration framewor
ii imagemagick 6.2.4.5-0.8 Image manipulation programs
ii libfreetype6 2.1.10-3 FreeType 2 font engine, shared library files
ii libx11-6 1.0.0-6 X11 client-side library
ii xfonts-100dpi 1.0.0-2 100 dpi fonts for X
ii xfs 1.0.1-4 X font server