Instead of being notified only when a host becomes unreachable, I'd also like to know when a host becomes reachable. One of my sites has two WAN connections and I'd like to be notified that the backup connection becomes active when it does.

Just to clarify. Do you mean that your backup connection remains inactive while the primary connection is operational, and it only activates if the primary connection goes down?
Or are they both operational, in which case as I understand, you would like to receive a notification that the backup connection is still operational when the primary one goes offline.

The backup only becomes active when the primary goes down. They are never both operational at the same time.

That's an interesting idea. I will run it by our programming team and see how fast we can implement it.

In a way you can already do that.

1. Create a new host group
2. Add the backup connection host (which is usually offline) to this group
3. Assign a contact that has "Repeats" options set to "None"
4. Enable the monitoring on the host

You will receive 1 notification saying that he host is offline, which is normal since the host is supposed to be offline. So simply disregard this notification. Now, when your main connection goes down and backup connection comes online you will receive a "Connection Restored" notification for the backup host just as you described.