Yesterday evening I let update-manager update gutsy. I hadn't done that for a couple of days, since it was the weekend and I didn't feel like battling with any problems an update might cause. The updating took a couple of iterations (I assume new updates arrived while the first one took place, or something like that), and at the end there was a "please reboot" icon in the panel. I didn't reboot, I just shut down. It was late and I wanted to sleep.
This morning, I turned on the laptop again, it booted fine, and I logged in. First problem: it played a login sound, but I'm pretty sure I had disabled that before. That's easy to fix, but then there was a second problem: the desktop background picture was shown, but nothing else: no panels, no desktop icons. I waited maybe a minute or two. Caps Lock did toggle the LED.
I checked things via the text mode virtual console, all the apps (gnome-panel, nautilus, metacity) were running.
I killed X with ctrl-alt-backspace, and logged back in. This time, same situation except there was alo no sound, and no desktop background. Rebooted, same thing (first login sound+background, second not). Tried the failsafe GNOME session, still the same.
Rebooted. Created a temporary account. Same behavior as before, except on the third or so try, it eventually came up with the panel and desktop icons. It took perhaps two minutes (guessing, didn't actually time it with a watch, since I was starting at the screen and trying to think what the problem might be).
Logged out, logged in with my own account. Now it worked. Rebooted. Logged in as myself again, and it complained that it couldn't start the GNOME settings service (gconfd, I assume), but that it would be started again on the next login. Logged out, and back in, and now it did work. Am writing this bug report from within that login session.
Today, my laptop still works, although I've had the "gconfd doesn't start" error at least once today, after updating and rebooting.