I got an interesting mail from AWS today.
One or more of your Amazon EC2 instances have been scheduled for a reboot in order to receive some patch updates. Most reboots complete within minutes, depending on your instance configuration. The instance(s) that will be rebooted and your scheduled reboot time(s) are listed below…
If you do want to manage your reboots for yourself, or simply want more information on the reboot process, please visit the Amazon EC2 Maintenance Help Page at: http://aws.amazon.com/maintenance-help/
I run my EC2 instances as part of a load-balanced cluster.I should take the instance out of the cluster, reboot it and then put it back into the cluster. Actually, I won’t but as we scale and have heavier usage I will have to manage this properly.
Dr James Bayley 
It’s worth noting that, if you follow their advice of rebooting your instances manually, the scheduled event icon doesn’t go away immediately. In fact, I’ve been waiting a few hours now and it’s still there on an instance I rebooted manually. AWS EC2 forums are awash with people waiting >24 hours for it the disappear, believing that it won’t and AWS will reboot it again. Which of course is annoying a few people who have to manually intervene with their instances when they don’t come back up on their own.
Thanks Danneh, that is really useful to know. James