I found a solution! (sort of)
EDIT: Nope, solution didn't work. Eventually I got another screen lockup, and have gotten several more since.
It was apparent after some searching that gnome-screensaver was the root of the issue (see https://bugs.launchpad.net/ubuntu/+s...er/+bug/762918 )
So, I uninstalled the package:
So far (48 hours and counting), I haven't had a single problem! (but I'll be sure to report back if I run into any issues)
sudo apt-get remove gnome-screensaver
Note that the "Turn screen off when inactive for x minutes" settings are in "Brightness and Lock" and don't require the gnome-screensaver packages, so I still get a black screen as a "screensaver".