hfs11 began crashing periodically beginning at 5:30pm on Sunday May 22, seemingly due to some memory problems -- at least that's what the logs seem to suggest. Of course, it had to wait until it was mostly full of users (including the CIO), because that's just the way these things go.
The memory was replaced around 9am. We'll see how she works now.
update: sun concurred that it was the memory.