We've been having spontaneous reboots on several machines in the last two months or so.
We've had an electrician double-check the power we're getting and all is well.
Looking in to potential computer-based issues I discover that many people experience this kind of thing with SSDs on Sabayon, Debian, Ubuntu, CentOS, and likely other distros. There do not seem to be any real solutions, but suggestions include the usual:
1) Get the latest firmware for the drive. Right now we have at least 2 different model of SSD in our machines (haven't checked Martin's yet): OCZ Vertex (96GB running f/w v1.6) and Vertex2 (115GB running f/w v1.29 or 1.33). Firmware and general info on OCZ drives can be found here. Firmware is here
2) Adjust fstab and /etc/rc.local like this

I'll come back to this next week.

<egg_on_face>
Figured it out. The problem was I stupidly changed the security update config to reboot after a security update gets done. It explains everything. The file has been edited, the package updated. Now I wait with my fingers crossed...
</egg_on_face>