Some automatic updates had been downloaded once I'd logged in, and I now had to restart. The day before I had a similar dialog which had disabled the Restart Later but didn't have a countdown, so I moved the dialog to the side and carried on (luckily it wasn't system modal). However a five minute timer is just amazingly bad: did installing the security fix break the system so much that it has to be rebooted now, and applications will start to crash and die shortly?

NP: Maxinquaye, Tricky