This isn't a bug in Grml's zshrc but instead an bug in Zsh upstream, see e.g.:

http://www.zsh.org/mla/workers/2011/msg00235.html
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=353863

So update your Zsh version (and/or bug Ubuntu that they provide a fixed package, the one in 
Debian/unstable is known to be working).

regards,
-mika-

On Ubuntu 11.04, using the .zshrc provided on http://git.grml.org/f/grml-etc-core/etc/zsh/zshrc causes my zsh to not react to any user input anymore if I try to tab-complete 'ls'

Reproduction procedure:
Start Ubuntu 11.04
Install zsh 
Run a Terminal (default shell still /bin/bash)
bash$ wget -O ~/.zshrc http://git.grml.org/f/grml-etc-core/etc/zsh/zshrc 
bash$ /bin/zsh
zsh$ ls[TAB]

Expected behavior:
Auto-Completion menu should pop up for commands relevant for string 'ls'

Actual behavior:
Cursor blinks, but no reaction to user input whatsoever, not even Ctrl+C. Only solution is to kill the terminal window.

This also happens on my x86 Gentoo machine, reproduction procedure is the same. Notice that this does not happen if I type ls[Space][Tab], only on ls[Tab]

Addittional Info:
$ zsh --version
zsh 4.3.11 (x86_64-unknown-linux-gnu)
$ uname --all
Linux sf 2.6.38-11-generic #50-Ubuntu SMP Mon Sep 12 21:17:25 UTC 2011 x86_64 x86_64 x86_64 GNU/Linux