Kernel panic, followed by corruption in the file system.
FSCK did not resolve the issue and the site was restored from the previous evenings backup.
Fortunately, this was all on the host and all I had to do was wait.
We did lose a couple registrations but I've e-mailed the concerned parties and hopefully they try again.
If not, it was a great way to weed out a couple spambots.
Thanks!