If you've ever lost your keys in the house but have your phone, then PhoneHalo might be what you're looking for -- apart from your keys. The Bluetooth device attaches to something (typically keys) and allows you to "ping" the device via an app on your iPhone. It works the other way as well, although no word on support for backgrounding or push notifications. The video has a brief description, although the app isn't available on the iPhone yet.
In the video, the narrator mentions a heavy social component, but I'm not really keen on telling my friends on Twitter and Facebook that a) I've lost my keys or b) where they are. Nevertheless, if you are a chronic key loser this could be your new best friend. Currently available for BlackBerry and Android, the iPhone version is slated for September 30 and you can (somehow) pre-order it now for US$1. You can order one PhoneHalo device (plus software for the currently-supported phones) for $59.95. Order 2 devices for $95.95 or 3 for $139.95. Or just tack a nail to the wall so you can hang your keys up.