Complete and unexpected hard-lock on authentication

Question

Hello. I've recently experienced my first hard-lock on my MacBook Pro. I'm running 10.6.8, and I can't seem to figure out what cuased it. It's making me very worried for the health and stability of my system.
What happened was this: I use password protection on my screen saver, and I do not put my laptop into sleep mode. Instead I use a hotcorner to activate the screen saver when I walk away from the machine, and then reauthenticate when I come back. After about ~30 min away from the keys, I came back and tried to authenticate, only to find that the password input window was not accepting keyboard input. Eventually I realized that the system had hard locked for a reason I couldn't understand. I had to force the machine to power down, and then boot it back up. The reboot was slower than I'd expect (though maybe that was just my nerves and imagination). In the hour or so since then, it's not repeated itself, even when trying to replicate the circumstances of the lockup.
The console messages didn't help much, but perhaps someone else can make more sense of them than I can. From what I can gather, a backup started, and then the ethernet port went down. My VPN client (which wasn't running or connected) then tried to switch interfaces to see if other internet connections were available - despite my not being connected to the VPN or running the client.
Here's the relevent time stamped log entries between the first notice of hard-locking and rebooting. I've redacted computer, drive, and host names, as well as mac addresses and IP addresses. I'm not sure what to make of it. Any help would be greatly appreciated. I'm not sure if this was a fluke, or a harbringer of something more sinister.
12/12/11 4:33:09 PM     com.apple.backupd[1322]     Starting standard backup
12/12/11 4:33:14 PM     com.apple.backupd[1322]     Backing up to: /Volumes/[REDACTED]/Backups.backupdb
12/12/11 4:33:21 PM     com.apple.backupd[1322]     No pre-backup thinning needed: 3.32 GB requested (including padding), 38.24 GB available
12/12/11 4:34:47 PM     kernel     Ethernet [AppleBCM5701Ethernet]: Link down on en0
12/12/11 4:34:47 PM     kernel     CiscoVPN : attempting to attach to all available ethernet interfaces.
12/12/11 4:34:47 PM     kernel     CiscoVPN : checking if we are already attached to interface: en1
12/12/11 4:34:47 PM     kernel     CiscoVPN : checking if we are already attached to interface: en0
12/12/11 4:34:47 PM     kernel     CiscoVPN : attempting to attach to all available ethernet interfaces.
12/12/11 4:34:47 PM     kernel     CiscoVPN : checking if we are already attached to interface: en1
12/12/11 4:34:47 PM     kernel     CiscoVPN : checking if we are already attached to interface: en0
12/12/11 4:34:47 PM     acvpnagent[46]     The network interface for the VPN connection has gone down.
12/12/11 4:34:47 PM     acvpnagent[46]     Function: logInterfaces File: RouteMgr.cpp Line: 2105 Invoked Function: logInterfaces Return Code: 0 (0x00000000) Description: IP Address Interface List:
12/12/11 4:34:47 PM     acvpnagent[46]     Function: applyHostConfigForNoVpn File: MainThread.cpp Line: 8446 No network interface is available, will not apply the noVPN host configuration.
12/12/11 4:34:47 PM     acvpnagent[46]     Current network state: No network interface
12/12/11 4:34:47 PM     acvpnagent[46]     Function: applyHostConfigForNoVpn File: MainThread.cpp Line: 8446 No network interface is available, will not apply the noVPN host configuration.
12/12/11 4:34:47 PM     configd[13]     setting hostname to "[REDACTED].local"
12/12/11 4:34:47 PM     configd[13]     network configuration changed.
12/12/11 4:34:55 PM     com.apple.backupd[1322]     Copied 902 files (1.2 MB) from volume [REDACTED].
12/12/11 4:34:56 PM     com.apple.backupd[1322]     No pre-backup thinning needed: 3.32 GB requested (including padding), 38.23 GB available
12/12/11 4:34:57 PM     com.apple.Finder[174]     Failed to send message to backup server: error = -2
12/12/11 4:34:58 PM     mDNSResponder[29]      19: Could not write data to client after 10 seconds, 3 replies waiting
12/12/11 4:35:03 PM     com.apple.backupd[1322]     Copied 153 files (491 KB) from volume [REDACTED].
12/12/11 4:35:04 PM     com.apple.backupd[1322]     Starting post-backup thinning
12/12/11 4:35:07 PM     mDNSResponder[29]      19: Could not write data to client after 20 seconds, 3 replies waiting
12/12/11 4:35:11 PM     com.apple.backupd[1322]     Deleted backup /Volumes/[REDACTED]/Backups.backupdb/[REDACTED]/2011-12-11-155148: 38.24 GB now available
12/12/11 4:35:11 PM     com.apple.backupd[1322]     Post-back up thinning complete: 1 expired backups removed
12/12/11 4:35:11 PM     com.apple.backupd[1322]     Backup completed successfully.
12/12/11 4:35:17 PM     mDNSResponder[29]      19: Could not write data to client after 30 seconds, 3 replies waiting
12/12/11 4:35:28 PM     mDNSResponder[29]      19: Could not write data to client after 40 seconds, 3 replies waiting
12/12/11 4:35:38 PM     mDNSResponder[29]      19: Could not write data to client after 50 seconds, 3 replies waiting
12/12/11 4:35:38 PM     acvpnagent[46]     A new network interface has been detected.
12/12/11 4:35:38 PM     acvpnagent[46]     Function: logInterfaces File: RouteMgr.cpp Line: 2105 Invoked Function: logInterfaces Return Code: 0 (0x00000000) Description: IP Address Interface List: [REDACTED]
12/12/11 4:35:38 PM     kernel     Ethernet [AppleBCM5701Ethernet]: Link up on en0, 1-Gigabit, Full-duplex, No flow-control, Debug [REDACTED]
12/12/11 4:35:38 PM     kernel     CiscoVPN : attempting to attach to all available ethernet interfaces.
12/12/11 4:35:38 PM     kernel     CiscoVPN : checking if we are already attached to interface: en1  12/12/11 4:35:38 PM     kernel     CiscoVPN : checking if we are already attached to interface: en0

Answer 1

I saw one very recent question where Test Pilot proved to be a problem.
In due course, you'll probably need to work your way through this -
https://support.mozilla.com/en-US/kb/Troubleshooting%20extensions%20and%20themes