As near as I can tell, some update or tweak in the way X works has invalidated the configuration of my mouse via hal. This is definitely getting tiring. I have had to work around it by putting this in my .xinitrc:

xinput set-int-prop '"Logitech USB Trackball"' 'Evdev Wheel Emulation Button' 8 8
xinput set-int-prop '"Logitech USB Trackball"' 'Evdev Wheel Emulation' 8 1

If it starts to work differently, I’ll update, again.