Is there a way I can set up my linode so that I get an automatic email whenever it goes down or has to reboot? I took a look at Alert Notification Settings but nothing there seems to relate to boots/reboots.

In addition to the native alerts/notifications that linode does, you can sign up for a free account with Pingdom. That'll give you the ability to monitor a single host and notify you if it goes down.

I've seen other suggestions in these forums that you have an email automatically sent from your Linode when it boots.

...such as http://www.linode.com/forums/viewtopic.php?t=3735#17824, and the cron entry above.