Anyone tracking Debian unstable should be aware of this bug in xserver-xorg-input-kbd. Essentially, the old ‘keyboard’ driver has been replaced by ‘kbd’ but the package doesn’t alter your xorg.conf file when you install it.
You’ll probably be totally unaware of this until you reboot your computer, as I did, and find that X is not working … most likely, at the least convenient moment.
The solution is quite simple. Replace:
…in your xorg.conf file.