Okay, so between various life things I hadn't spent much time with MB for a few weeks. When I started it up just now, I discovered that my mouse wheel scrolling was reversed from the usual. Down was scrolling up and up was scrolling down.
Thankfully, it occurred to me to go check my mouse settings in the control panel. Awhile back I had switched from "scroll x lines" to "scroll 1 screen". Switching back (and restarting MB) fixed the problem.
Can anyone else reproduce this?