Thanks for report.  The bug (package names concatenated without any delimiter)
was introduced with my optimization for speed.  This patch seems to fix this.

--- /usr/share/zsh/functions/Completion/Debian/_rpm_packages-   2003-11-06
18:33:46 +0300
+++ /usr/share/zsh/functions/Completion/Debian/_rpm_packages    2004-01-14
15:52:00 +0300
@@ -20,7 +20,7 @@
       _cache_invalid RPMS_installed ) && ! _retrieve_cache RPMS_installed;
   then
     _rpm_packages_cache_installed=(
-      ${(z)$(rpm -qa --qf '%{NAME}')}
+      ${(z)$(rpm -qa --qf '%{NAME}\n')}
     )
     _store_cache RPMS_installed _rpm_packages_cache_installed
   fi

Fixed in zsh-4.1.1-alt4.