problems generating squeeze image with live-build 3.x from squeeze

[Marco Pantaleoni]

I'm building a Debian Squeeze with live-build 3.0~a35+20111013.073832~70w
(from a Debian Squeeze using live-* packages from wheezy-snapshots).

I'm doing this:

# lb config --distribution="squeeze" \
        --archive-areas="main contrib non-free" \
        --linux-flavours="486" \
        --architectures i386 \
        --package-lists minimal \
        --binary-images hdd \
        --cache true \
        --cache-indices true \
        --cache-packages true \
        --apt-indices false \
        --memtest none \
        --apt-recommends false \
        --includes none \
        --bootappend-live="timezone=Europe/Rome hostname=custom
       locale=en_US.utf8 keyb=en quiet" \
        --debian-installer="false" \
        --debian-installer-gui="false" \
        --hostname="custom" \
        --username="root" \
        --bootloader syslinux


# cat << 'EOF' > config/archives/live.list.chroot
# Debian Live (wheezy/testing snapshots)
deb http://live.debian.net/ wheezy-snapshots main contrib non-free
deb-src http://live.debian.net/ wheezy-snapshots main contrib non-free
EOF

# cat << 'EOF' > config/chroot_apt/preferences
Package: live-*
Pin: release n=wheezy
Pin-Priority: 950

Package: live-build live-boot live-boot-initramfs-tools live-config
 live-config-sysvinit syslinux-themes-debian-squeeze syslinux-themes-debian
Pin: release n=wheezy
Pin-Priority: 900

Package: *
Pin: release n=squeeze
Pin-Priority: 900

Package: *
Pin: release n=wheezy
Pin-Priority: 1
EOF

(these last commands loosely inspired by
http://lists.debian.org/debian-live/2011/09/msg00038.html)

# lb build

then the process completes, but without the single binary image file.
In the output I can see these lines:

...
Reading package lists... Done
W: GPG error: http://live.debian.net wheezy-snapshots Release: The
following signatures couldn't be verified because the public key is not
available: NO_PUBKEY 6F95B4996CA7B5A6
...
Reading package lists... Done
W: GPG error: http://live.debian.net wheezy-snapshots Release: The
following signatures couldn't be verified because the public key is not
available: NO_PUBKEY 6F95B4996CA7B5A6
...
...
E: Unable to locate package syslinux
E: Unable to locate package syslinux-themes-debian-squeeze
...

so, I believe that for some reason the apt pinning tricks didn't work as I
expected.
What am I doing wrong and how can I solve the problem?

Thanks in advance.
Marco

-- 
Marco Pantaleoni