I this on armel debian gnu/linux unstable system and since it does not look like a distro specific bug I thought I should attach a copy here:

Package: python-gobject
Version: 2.14.2-1
Severity: normal

Steps to reproduce:
1) cat > testcase.py <<EOF
import gobject, os
gobject.spawn_async( [ "/bin/sleep", "4" ],
                     child_setup = None)
EOF
2) strace -e write -f python a.py

Expected results:
2) gobject forks and exec's sleep

Actual results:
2) gobject forks but then tries to call child_setup function even thought it is None
   as can be seen from strace output

Process 11653 attached (waiting for parent)
Process 11653 resumed (parent 11652 ready)
Process 11654 attached (waiting for parent)
Process 11652 suspended
Process 11654 resumed (parent 11653 ready)
[pid 11654] write(2, "TypeError"..., 9TypeError) = 9
[pid 11653] write(6, "\206-\0\0"..., 4 <unfinished ...>
[pid 11654] write(2, ": "..., 2: )        = 2
[pid 11654] write(2, "'NoneType' object is not callable"..., 33'NoneType' object is not callable) = 33
[pid 11654] write(2, "\n"..., 1
)        = 1
[pid 11653] <... write resumed> )       = 4
Process 11652 resumed
Process 11653 detached
[pid 11652] --- SIGCHLD (Child exited) @ 0 (0) ---
Process 11654 detached


-- System Information:
Debian Release: 5.0
  APT prefers unstable
  APT policy: (500, 'unstable')
Architecture: armel (armv4tl)

Kernel: Linux 2.6.28-GTA02_lindi-andy-tracking_b8b36e5ec3db71d5-mokodev (PREEMPT)
Locale: LANG=C, LC_CTYPE=fi_FI (charmap=ISO-8859-1)
Shell: /bin/sh linked to /bin/dash

Versions of packages python-gobject depends on:
ii  libc6                         2.7-18     GNU C Library: Shared libraries
ii  libffi5                       3.0.7-1    Foreign Function Interface library
ii  libglib2.0-0                  2.16.6-1   The GLib library of C routines
ii  python                        2.5.2-3    An interactive high-level object-o
ii  python-support                0.8.7      automated rebuilding support for P
ii  python2.5                     2.5.2-15   An interactive high-level object-o

python-gobject recommends no packages.

Versions of packages python-gobject suggests:
pn  python-gobject-dbg            <none>     (no description available)