For most iOS client / server applications, push notifications are negotiated through an intermediary service provider, such as Urban Airship. The service provider exposes APIs to register a device token, as well as APIs to send push notifications to devices meeting some specified criteria.
Orbiter is a small library that provides simple interfaces to register (and unregister) for Push Notifications with Urban Airship (without needing to include their SDK), as well as Helios apps.