I have killed the last application I used (firefox). - Xorg still at 100%. One CPU is 100% (2.5G) the other 1.5G.
My desktop is Xfce4.

Here are the gory details:

Linux cosmo 2.6.32-24-generic #42-Ubuntu SMP Fri Aug 20 14:21:58 UTC 2010 x86_64 GNU/Linux
00:00.0 Host bridge: Intel Corporation 82G35 Express DRAM Controller (rev 03)
00:02.0 VGA compatible controller: Intel Corporation 82G35 Express Integrated Graphics Controller (rev 03)
00:02.1 Display controller: Intel Corporation 82G35 Express Integrated Graphics Controller (rev 03)
00:19.0 Ethernet controller: Intel Corporation 82566DC Gigabit Network Connection (rev 02)
00:1a.0 USB Controller: Intel Corporation 82801H (ICH8 Family) USB UHCI Controller #4 (rev 02)
00:1a.1 USB Controller: Intel Corporation 82801H (ICH8 Family) USB UHCI Controller #5 (rev 02)
00:1a.7 USB Controller: Intel Corporation 82801H (ICH8 Family) USB2 EHCI Controller #2 (rev 02)
00:1b.0 Audio device: Intel Corporation 82801H (ICH8 Family) HD Audio Controller (rev 02)
00:1c.0 PCI bridge: Intel Corporation 82801H (ICH8 Family) PCI Express Port 1 (rev 02)
00:1c.1 PCI bridge: Intel Corporation 82801H (ICH8 Family) PCI Express Port 2 (rev 02)
00:1c.2 PCI bridge: Intel Corporation 82801H (ICH8 Family) PCI Express Port 3 (rev 02)
00:1d.0 USB Controller: Intel Corporation 82801H (ICH8 Family) USB UHCI Controller #1 (rev 02)
00:1d.1 USB Controller: Intel Corporation 82801H (ICH8 Family) USB UHCI Controller #2 (rev 02)
00:1d.2 USB Controller: Intel Corporation 82801H (ICH8 Family) USB UHCI Controller #3 (rev 02)
00:1d.7 USB Controller: Intel Corporation 82801H (ICH8 Family) USB2 EHCI Controller #1 (rev 02)
00:1e.0 PCI bridge: Intel Corporation 82801 PCI Bridge (rev f2)
00:1f.0 ISA bridge: Intel Corporation 82801HB/HR (ICH8/R) LPC Interface Controller (rev 02)
00:1f.2 SATA controller: Intel Corporation 82801HB (ICH8) 4 port SATA AHCI Controller (rev 02)
00:1f.3 SMBus: Intel Corporation 82801H (ICH8 Family) SMBus Controller (rev 02)
03:00.0 IDE interface: JMicron Technology Corp. JMB368 IDE controller
04:05.0 FireWire (IEEE 1394): Agere Systems FW322/323 (rev 70)

top - 16:27:04 up 4 days, 16:01, 15 users, load average: 1.10, 1.07, 0.95
Tasks: 221 total, 2 running, 217 sleeping, 0 stopped, 2 zombie
Cpu(s): 0.5%us, 50.5%sy, 0.0%ni, 49.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 8185024k total, 7343296k used, 841728k free, 985328k buffers
Swap: 7815612k total, 932k used, 7814680k free, 4028856k cached

  PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
30044 root 20 0 338m 106m 43m R 100 1.3 152:33.47 Xorg
 1310 doug 20 0 672m 207m 32m S 2 2.6 63:57.46 midori
    1 root 20 0 23840 2008 1268 S 0 0.0 0:00.69 init
    2 root 20 0 0 0 0 S 0 0.0 0:00.01 kthreadd
    3 root RT 0 0 0 0 S 0 0.0 0:00.78 migration/0

X.Org X Server 1.7.6
Release Date: 2010-03-17
X Protocol Version 11, Revision 0
Build Operating System: Linux 2.6.24-27-server x86_64 Ubuntu
Current Operating System: Linux cosmo 2.6.32-24-generic #42-Ubuntu SMP Fri Aug 20 14:21:58 UTC 2010 x86_64
Kernel command line: BOOT_IMAGE=/vmlinuz-2.6.32-24-generic root=UUID=e201f17f-77cc-45b9-9665-8719827b9805 ro quiet splash
Build Date: 21 July 2010 01:03:39PM
xorg-server 2:1.7.6-2ubuntu7.3 (For technical support please see http://www.ubuntu.com/support)
Current version of pixman: 0.16.4
        Before reporting problems, check http://wiki.x.org
        to make sure that you have the latest version.
sudo strace -p 30044
--- SIGALRM (Alarm clock) @ 0 (0) ---
rt_sigreturn(0xe)                       = 50800
--- SIGALRM (Alarm clock) @ 0 (0) ---
rt_sigreturn(0xe)                       = 50800
--- SIGALRM (Alarm clock) @ 0 (0) ---
rt_sigreturn(0xe)                       = 50800
--- SIGALRM (Alarm clock) @ 0 (0) ---
rt_sigreturn(0xe)                       = 50800
--- SIGALRM (Alarm clock) @ 0 (0) ---
rt_sigreturn(0xe)                       = 50800
--- SIGALRM (Alarm clock) @ 0 (0) ---
rt_sigreturn(0xe)                       = 50800
--- SIGALRM (Alarm clock) @ 0 (0) ---
rt_sigreturn(0xe)                       = 50800
Repeats indefinitely.