Sure, push notification is nothing new for iPhone users. But when was the last time that you received notification that real-live mail -- the kind made of atoms, not bits -- has shown up in your mailbox?
One of the new evil geniuses at Make Magazine online, Matt Richardson, decided he'd like to get notification when the flesh-and-blood mailman delivers something to his mailbox. Using a standard USPS-approved mailbox, he installed a snap-action switch that signals when the mailbox door has been opened. That switch is connected via wire (yeah, I was also surprised that it wasn't wireless) to an Arduino. Some simple code watches for the mailbox door to be opened and closed, and then grabs a piece of PHP code from a web server that also has to be running.
The PHP and web server are necessary since Matt is using the Prowl: Growl Client app (US$2.99) to get the push notifications. Prowl requires an SSL connection, which that Arduino can't make. As Matt notes, the end product is something that can send push notifications to your iPhone whenever some physical state changes -- when the garage door is left open, when home power usage exceeds a certain level, etc... Any switch or sensor connected to the Arduino can send a push alert.
There's video of the construction and use of the setup on page 2.