Within the last few days, my n900's proximity sensor has stopped working with the "Shortcutd" program, which I use to bring up the Dashboard/task-switcher. It is still working for purposes of blanking the screen when in a phone call. I am running the latest firmware and the only other application I can think of that I have installed which might mess with this underlying subsystem is Simple Brightness Applet.

It was really strange to me how spontaneously this stopped working for no easily apparent reason.