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Support for Passbook is also built into OSX Mountain Lion (10.8.2). Pass files with the pkpass file extension will open in a preview window:

Clicking on the Add to Passbook button will place the Pass in the Passbook associated with the iCloud account set up in OSX system preferences.

The OSX Mail app and Safari also support embedded Passes.

When building a Pass, you can specify a relevant time and up to 10 relevant locations that will trigger a message to be displayed on the lock screen. The message looks similar to a push notification, however a Pass notification is less intrusive. When it is relevant to display, it doesn't vibrate the iPhone and it doesn't wake up the screen. The notification only becomes visible when the phone wakes up from sleep:

The option to specify relevant times and locations, and how far from the location the notification is triggered, is determined by the Pass type, as we will see later.