Sorry for the ambuguity - I'm admittedly not an expert on OS X internals. The effect I'm experiencing is an immediate reboot of the machine - no intermediate screen or other error indication, just an instant reboot (gray screen, boot sound, spinner and eventually back to my desktop). After the reboot, as the Mac is coming back up, I see a crash report dialog. One of the lines in that report mentions a "kernel panic" from an unknown thread.