I have received the same error but only for my regular hard line (eth0), I have switched off the wireless (eth1) in the BIOS. I took this information from the messages log file located in /var/log/   This also affects both Ubuntu 7.04 and Xubuntu 7.04 64bit versions for AMD64, I have tried both. When it crashes, my system locks up solid, mouse does not work, system load usually max's out and processor is at 100%. I have even unplugged my internet connection to see if it clears up but it doesn't. Most of the time I have to manually press the main power switch to get the system to shut off. I have been able to log in shell or tty and I get the error showing up on screen. There too, ctrl+alt+del, I have to press 3 or more times to get the system to reboot, tried logging in and out but that is hopeless.  This is a nasty problem. 

Sep  4 00:14:04 thomas-laptop syslogd 1.4.1#20ubuntu4: restart.
Sep  4 00:24:57 thomas-laptop dhcdbd: dhco_parse_option_settings: bad option setting: old_server_name =  
Sep  4 00:25:00 thomas-laptop dhcdbd: dhco_parse_option_settings: bad option setting: old_server_name =  
Sep  4 00:25:00 thomas-laptop dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth0 for sub-path eth0.dbus.get.domain_name
Sep  4 00:25:00 thomas-laptop dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth0 for sub-path eth0.dbus.get.nis_domain
Sep  4 00:25:00 thomas-laptop dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth0 for sub-path eth0.dbus.get.nis_servers
Sep  4 08:08:50 thomas-laptop dhcdbd: dhco_parse_option_settings: bad option setting: old_server_name =  
Sep  4 08:08:50 thomas-laptop dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth0 for sub-path eth0.dbus.get.domain_name
Sep  4 08:08:50 thomas-laptop dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth0 for sub-path eth0.dbus.get.nis_domain
Sep  4 08:08:50 thomas-laptop dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth0 for sub-path eth0.dbus.get.nis_servers
Sep  4 08:12:32 thomas-laptop syslogd 1.4.1#20ubuntu4: restart.
Sep  4 08:12:36 thomas-laptop dhcdbd: Started up.
Sep  4 08:12:39 thomas-laptop dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth0 for sub-path eth0.dbus.get.reason
Sep  4 08:12:39 thomas-laptop dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth0 for sub-path eth0.dbus.get.reason
Sep  4 08:12:45 thomas-laptop dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth0 for sub-path eth0.dbus.get.domain_name
Sep  4 08:12:45 thomas-laptop dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth0 for sub-path eth0.dbus.get.nis_domain
Sep  4 08:12:45 thomas-laptop dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth0 for sub-path eth0.dbus.get.nis_servers
Sep  4 14:26:08 thomas-laptop dhcdbd: Started up.
Sep  4 14:26:11 thomas-laptop dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth0 for sub-path eth0.dbus.get.reason
Sep  4 14:26:11 thomas-laptop dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth0 for sub-path eth0.dbus.get.reason
Sep  4 14:26:15 thomas-laptop dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth0 for sub-path eth0.dbus.get.domain_name
Sep  4 14:26:15 thomas-laptop dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth0 for sub-path eth0.dbus.get.nis_domain
Sep  4 14:26:15 thomas-laptop dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth0 for sub-path eth0.dbus.get.nis_servers
Sep  4 19:20:07 thomas-laptop syslogd 1.4.1#20ubuntu4: restart.
Sep  4 19:20:11 thomas-laptop dhcdbd: Started up.
Sep  4 19:20:14 thomas-laptop dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth0 for sub-path eth0.dbus.get.reason
Sep  4 19:20:14 thomas-laptop dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth0 for sub-path eth0.dbus.get.reason
Sep  4 19:20:20 thomas-laptop dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth0 for sub-path eth0.dbus.get.domain_name
Sep  4 19:20:20 thomas-laptop dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth0 for sub-path eth0.dbus.get.nis_domain
Sep  4 19:20:20 thomas-laptop dhcdbd: message_handler: message handler not found under /com/redhat/dhcp/eth0 for sub-path eth0.dbus.get.nis_servers