We believe that your issue has been closed by the upload of
Version 0.18 of grml-debootstrap from Michael Prokop <mika@grml.org>.
The explanation is attached below

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Format: 1.8
Date: Sat, 30 Aug 2008 02:01:14 +0200
Source: grml-debootstrap
Binary: grml-debootstrap
Architecture: source all
Version: 0.18
Distribution: unstable
Urgency: low
Maintainer: Michael Prokop <mika@grml.org>
Changed-By: Michael Prokop <mika@grml.org>
Description: 
 grml-debootstrap - wrapper around debootstrap for installing plain Debian via grml
Changes: 
 grml-debootstrap (0.18) unstable; urgency=low
 .
   The "development sponsored by Sipwise GmbH" release.
 .
   * Provide interactive configuration dialog.
   * Initial support for SW-RAID.
   * Use stages based on target information, this gives us
     the possibility to run multiple instance of grml-debootstrap.
     [Closes: issue510]
   * Use policy-rc.d inside chroot to avoid startup of daemons.
     [Closes: issue509]
   * Do not activate /sys by default anymore in /etc/fstab.
   * Copy /etc/network/interfaces from running system to target
     installation.
   * Setting password via passwd does not work, use chpaswd instead.
   * Remove stages after successful execution. [Closes: issue513]
   * Make sure the configuration file inside the chroot provides
     the same configuration as specified on the cmdline (thanks gebi
     for reporting).
   * Copy system's /etc/hosts to the target system.
   * Move stages to /var/cache/grml-debootstrap (thanks for the idea,
     gebi).
   * Install devices using our own device tarball instead of running
     MAKEDEV (which is just an ungly fork bomb).
   * Install MTA postfix by default to avoid installation of unwanted
     packages like citadel in lenny.
   * Allow use of comments in file /etc/debootstrap/packages.
   * Replace initrd-tools with initramfs-tools.
   * Run aptitude with --without-recommends option.
   * Run grub-install using --no-floppy, as requested by gebi.
   * Support setting hostname via cmdline.
   * Close issue in grml's BTS which showed up during development in hg:
     Fix handling of config file inside chroot, thanks for spotting, gebi!
     [Closes: issue512]
   * Update debian/ to make lintian happy.
Checksums-Sha1: 
 87e9aeb16837255681d0c20b88e08779ff7ea178 946 grml-debootstrap_0.18.dsc
 6a7d3735dd5d982d979bce09c6b74f189f2504ce 129965 grml-debootstrap_0.18.tar.gz
 0cab3f36ea55bb61f4d476081e9e88f6b414b374 103046 grml-debootstrap_0.18_all.deb
Checksums-Sha256: 
 3cf038f081edb37faa0a9f88a622adb5839c7df6f9e9c449d01d67d1c15e4497 946 grml-debootstrap_0.18.dsc
 a7ebe36844944525d797fc8231b8bca5c1b869c63367c55cf5f7e4bdd8363647 129965 grml-debootstrap_0.18.tar.gz
 8ce04c66fabab8fd7763915c8c2a5602c53d5347eec95304da117543a6ed1ef5 103046 grml-debootstrap_0.18_all.deb
Files: 
 64b7b78b7b203872c7195ecf4e4183d9 946 grml optional grml-debootstrap_0.18.dsc
 3c4e797b9c9324f35ddd396033fdebae 129965 grml optional grml-debootstrap_0.18.tar.gz
 e15c716e6464cc403d01644fbb7eb0f9 103046 grml optional grml-debootstrap_0.18_all.deb

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.9 (GNU/Linux)

iD4DBQFIuI5Y2N9T+zficugRAsrpAJiOTZ4y8RFHi2oUDOpUGcIZpsvbAJ9N26lA
LuAkaPSajqjogMNSSYPMrw==
=Gidp
-----END PGP SIGNATURE-----

also tried without approx in the middle:

# grml-debootstrap -t /dev/sda -r lenny

doesn't change the behaviour

# chroot
# cat /etc/apt/sources.list
deb http://192.168.10.1:9999/debian etch main contrib non-free

grml-debootstrap version 0.17
grml-medium_sid_20080827

# grml-debootstrap -t /dev/sda1 -r lenny -m http://192.168.10.1:9999/debian
results in a messed up lenny install from debootstrap downgraded to etch (from 
grml-debootstrap?)
# cat /etc/apt/sources.list
deb http://192.168.10.1:9999/debian etch main contrib non-free


# debootstrap lenny /tmp/foo http://192.168.10.1:9999/debian
results in a clean lenny install
# cat /etc/apt/sources.list
deb http://192.168.10.1:9999/debian lenny main