This bug has been reported on Ubuntu Launchpad as: 
https://launchpad.net/bugs/407976

from the launchpad report:

I think it have something to do with Xfapplet (Applet for Xfce panel that allows Gnome applets to run inside of it).
Computer failed to shutdown and restarted gdm.
I went to console (Ctrl Alt F2) and killed with -9 xfapplet processes.
After that, you can log in and later did shutdown from gdm menu.


Architecture: amd64

DistroRelease: Ubuntu 8.04

Package: xfce4-session 4.4.2-2ubuntu2.1

PackageArchitecture: amd64

Signal: 11
SourcePackage: xfce4-session
StacktraceTop:
 ?? () from /lib/libc.so.6
 ?? () from /lib/libc.so.6
 malloc () from /lib/libc.so.6
 g_malloc () from /usr/lib/libglib-2.0.so.0
 g_string_chunk_insert_len ()
Title: xfce4-session crashed with SIGSEGV in malloc()
Uname: Linux 2.6.24-24-generic x86_64
UserGroups: adm admin audio cdrom dialout dip fax floppy fuse kvm lpadmin sambashare scanner tape vboxusers video


It might be reproduced with this actions:
Log into Xfce on 8.04 LTS 64bit , use Xfapplet to use multiple Gnome applets.
Log out , log in, log out again . Log into console and with top see multiple Xfe- applets processes, using 100% cou time.
If not killed, next time one logs in, no applet under xfapplet is started, but they run from previous session and use 100%cpu untill killed. Then, one can log out and log in and everything is fine, till next time.
Problem is on LTS that will soon be replaced with news Lts
and I have a few (xfce keyboard applet at least) that I add by myself. Many libraries on machine are also upgraded to newer versions so it is not machine that is in LTS-only state, because I installed many additional programs over time (that might be normal I suppose).
So truely, reproducing on other, LTS-only machine is truly questionable. I am betting on Xfapplet but not sure.

Stacktrace: 
http://launchpadlibrarian.net/29808465/Stacktrace.txt