Gah, I didn't even think to check here before I upgraded to 3.1. Hacking the Kindle was one of the very first things I did, and I'm so used to adding my own screensavers that I forgot it wasn't a built-in option. Now my "screensaver" is just a blank screen. Everything works fine, but I'm missing both the original and my custom pictures.
I guess I'll know better next time.