I don't know if this is a 'full backtrace' but that's what I was able to get out so far. I also included trace.

kurppa@el-bond:~$ less /etc/X11/xorg.conf
Section "Device"
        Identifier      "Configured Video Device"
EndSection
 
Section "Monitor"
        Identifier      "Configured Monitor"
EndSection
 
Section "Screen"
        Identifier      "Default Screen"
        Monitor         "Configured Monitor"
        Device          "Configured Video Device"
EndSection
 
Section "ServerFlags"
        Option "NoTrapSignals" "true"
EndSection




kurppa@el-bond:~$ sudo gdb /usr/bin/Xorg
[sudo] password for kurppa: 
GNU gdb 6.8-debian
Copyright (C) 2008 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "i486-linux-gnu"...
(gdb) run
Starting program: /usr/bin/Xorg 
[Thread debugging using libthread_db enabled]
 
X.Org X Server 1.5.2
Release Date: 10 October 2008
X Protocol Version 11, Revision 0
Build Operating System: Linux 2.6.24-19-server i686 Ubuntu
Current Operating System: Linux el-bond 2.6.27-7-generic #1 SMP Tue Nov 4 19:33:20 UTC 2008 i686
Build Date: 24 October 2008  08:00:16AM
xorg-server 2:1.5.2-2ubuntu3 (<email address hidden>) 
        Before reporting problems, check http://wiki.x.org
        to make sure that you have the latest version.
Module Loader present
Markers: (--) probed, (**) from config file, (==) default setting,
        (++) from command line, (!!) notice, (II) informational,
        (WW) warning, (EE) error, (NI) not implemented, (??) unknown.
(==) Log file: "/var/log/Xorg.0.log", Time: Tue Dec  9 09:44:13 2008
(==) Using config file: "/etc/X11/xorg.conf"
[New Thread 0xb7aab6b0 (LWP 6044)]
[tcsetpgrp failed in terminal_inferior: Operation not permitted]
 
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0xb7aab6b0 (LWP 6044)]
0x00000000 in ?? ()
(gdb) backtrace full
#0  0x00000000 in ?? ()
No symbol table info available.
#1  0xb79c7b07 in ?? () from /usr/lib/xorg/modules/drivers//radeon_drv.so
No symbol table info available.
#2  0xb799555a in ?? () from /usr/lib/xorg/modules/drivers//radeon_drv.so
No symbol table info available.
#3  0xb79984fc in ?? () from /usr/lib/xorg/modules/drivers//radeon_drv.so
No symbol table info available.
#4  0x080aac9f in InitOutput (pScreenInfo=0x8201200, argc=1, argv=0xbfa48bb4)
    at ../../../../hw/xfree86/common/xf86Init.c:749
        configured_device = <value optimized out>
        i = -1214279692
        j = 0
        k = <value optimized out>
        scr_index = <value optimized out>
        modulelist = <value optimized out>
        optionlist = (pointer *) 0x985ede0
        layout = (screenLayoutPtr) 0x98527d8
        screenpix24 = <value optimized out>
        pix24 = <value optimized out>
        pix24From = <value optimized out>
        autoconfig = <value optimized out>
#5  0x08071b19 in main (argc=1, argv=0xbfa48bb4, envp=0xbfa48bbc)
    at ../../dix/main.c:358
---Type <return> to continue, or q <return> to quit---
        i = <value optimized out>
        error = 136151028
        xauthfile = <value optimized out>
        alwaysCheckForInput = {0, 1}


Let me know if there's something more I can do.