Here is a backtrace:

Apr 11 02:32:46 localhost NetworkManager: ******************* START **********************************
Apr 11 02:32:47 localhost NetworkManager: Using host libthread_db library "/lib/tls/i686/cmov/libthread_db.so.1".
Apr 11 02:32:47 localhost NetworkManager: [Thread debugging using libthread_db enabled]
Apr 11 02:32:47 localhost NetworkManager: [New Thread -1211926848 (LWP 13973)]
Apr 11 02:32:47 localhost NetworkManager: 0xffffe410 in __kernel_vsyscall ()
Apr 11 02:32:47 localhost NetworkManager: 
Apr 11 02:32:47 localhost NetworkManager: Thread 1 (Thread -1211926848 (LWP 13973)):
Apr 11 02:32:47 localhost NetworkManager: #0  0xffffe410 in __kernel_vsyscall ()
Apr 11 02:32:47 localhost NetworkManager: #1  0xb7f11463 in __waitpid_nocancel () from /lib/tls/i686/cmov/libpthread.so.0
Apr 11 02:32:47 localhost NetworkManager: #2  0x0806ac32 in nm_logging_backtrace () at nm-logging.c:72
Apr 11 02:32:47 localhost NetworkManager: #3  0x0806ae05 in nm_signal_handler (signo=11) at nm-logging.c:166
Apr 11 02:32:47 localhost NetworkManager: #4  <signal handler called>
Apr 11 02:32:47 localhost NetworkManager: #5  0xb7f0ce47 in pthread_mutex_destroy ()
Apr 11 02:32:47 localhost NetworkManager:    from /lib/tls/i686/cmov/libpthread.so.0
Apr 11 02:32:47 localhost NetworkManager: #6  0x08059a3b in nm_device_802_11_wireless_dispose (object=0x0)
Apr 11 02:32:47 localhost NetworkManager:     at nm-device-802-11-wireless.c:3015
Apr 11 02:32:47 localhost NetworkManager: #7  0xb7c66eac in g_object_unref () from /usr/lib/libgobject-2.0.so.0
Apr 11 02:32:47 localhost NetworkManager: #8  0x08055e50 in nm_device_new (iface=0x80aa5d8 "wlan0", 
Apr 11 02:32:47 localhost NetworkManager: rg/freedesktop/Hal/devices/net_00_02_6f_06_a2_ea_0", 
Apr 11 02:32:47 localhost NetworkManager:     test_dev=0, test_dev_type=DEVICE_TYPE_UNKNOWN, app_data=0x8094110)
Apr 11 02:32:47 localhost NetworkManager:     at nm-device.c:205
Apr 11 02:32:47 localhost NetworkManager: #9  0x080658aa in nm_create_device_and_add_to_list (data=0x8094110, 
Apr 11 02:32:47 localhost NetworkManager:     udi=0x80aaf40 "/org/freedesktop/Hal/devices/net_00_02_6f_06_a2_ea_0", 
Apr 11 02:32:47 localhost NetworkManager:     iface=0x80aa5d8 "wlan0", test_device=0, 
Apr 11 02:32:47 localhost NetworkManager:     test_device_type=DEVICE_TYPE_UNKNOWN) at NetworkManager.c:127
Apr 11 02:32:47 localhost NetworkManager: #10 0x08065edf in nm_add_initial_devices (data=0x8094110)
Apr 11 02:32:47 localhost NetworkManager:     at NetworkManager.c:328
Apr 11 02:32:47 localhost NetworkManager: #11 0x0806655e in nm_hal_init (data=0x8094110) at NetworkManager.c:689
Apr 11 02:32:47 localhost NetworkManager: #12 0x080672ea in main (argc=1, argv=0xbff7e894) at NetworkManager.c:889
Apr 11 02:32:47 localhost NetworkManager: #0  0xffffe410 in __kernel_vsyscall ()
Apr 11 02:32:47 localhost NetworkManager: ******************* END **********************************

I tried to look at the code but couldn't come up with nothing (knowing about nothing about NM and glib). But I can see that #7 g_object_unref() is calling into #6 nm_device_802_11_wireless_dispose() with a NULL pointer which doesn't look good...