It looks like packagekitd crashes with a SIGABRT when trying to run a second instance of the daemon, when a previous instance is already running.

[root@ginger ~]# gdb /usr/sbin/packagekitd
(gdb) run --backend yum --verbose
[...]
** ERROR **: Error trying to start: Already running on this machine
aborting...

Program received signal SIGABRT, Aborted.
0x000000391d231f95 in raise () from /lib64/libc.so.6
(gdb) bt
#0  0x000000391d231f95 in raise () from /lib64/libc.so.6
#1  0x000000391d233a40 in abort () from /lib64/libc.so.6
#2  0x000000391fe374a5 in g_logv () from /lib64/libglib-2.0.so.0
#3  0x000000391fe37543 in g_log () from /lib64/libglib-2.0.so.0
#4  0x00000000004091d7 in main (argc=1, argv=0x7fffabfa2ed8) at pk-main.c:269

[rishi@ginger ~]$ ps -aux | grep packagekitd
Warning: bad syntax, perhaps a bogus '-'? See /usr/share/doc/procps-3.2.7/FAQ
root      5916  0.0  0.2  60180  2624 ?        Sl   20:36   0:00 /usr/sbin/packagekitd
root      5974  0.1  1.5 140720 16244 pts/1    S+   20:38   0:00 gdb /usr/sbin/packagekitd
root      6081  0.0  0.2  60184  2596 pts/1    Tl   20:45   0:00 /usr/sbin/packagekitd --backend yum --verbose
rishi     6104  0.0  0.0  82276   756 pts/0    S+   20:45   0:00 grep packagekitd

Installed packages:
gnome-packagekit-0.1.9-1.fc9
PackageKit-0.1.9-1.fc9
PackageKit-debuginfo-0.1.9-1.fc9
PackageKit-libs-0.1.9-1.fc9
yum-3.2.8-2.fc8