At set firing time, Xscreensaver fades to black like it's supposed to. But afterwards my display instantly pops back on, albeit with no mouse cursor, and my computer freezes up. I've tested this with just about every loaded module and with Guake turned on and off. What gives here?

What distro on what hardware, in particular, which video card)?

I haven't tested this, but you might also stop the xscreensaver daemon, then restart it from a terminal. Leave the terminal open (minimized if you wish); there might be some output to the terminal that sheds some light on what's going on behind the scenes.

Edit: Just tested. Start it with xscreensaver &. When I tested it, there was no output to the terminal under normal operation.

You might also check you log files, particularly /var/log/messages and /var/log/syslog. Again, a long shot.

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Now I'd like to know how to make Ubuntu default to Xscreensaver instead of GNOME screensaver at startup. More script editing (as if I didn't enjoy that kinda thing)?

Install Xscreensaver. It's been a while, but I think that removing Gnome screensaver might happen automatically when you do; otherwise, just remove it in the package manager. Then put xscreensaver in the startup applications.

If you want to put the preferences in your menu, that's "xscreensaver-demo."

One note: When you do a version upgrade, at least when I was using Ubuntu from 8.04 to 11.04, Ubuntu will replace xsceensaver (it's really quite obnoxious). Just follow the instructions above every six months or so and you will be okay.