Some companies are already providing (or planning to provide) a gateway to the APN:

These are:

• Urban Airship
• iLime
• AppNotify

These are in closed beta:

• Push.io
• iPushServer.com
• msgpush

Urban Airship and iLime also provide In App Purchase.

Or you can build your own: Open Source & Tutorials

• ApnsPHP: Apple Push Notification & Feedback Provider
• EasyAPNS: Open Source solution (PHP, MySQL)
• Tutorial on how to setup your own server in PHP
• Source code (in PHP), requires PHP with memcache
• Python wrapper for Apple Push Notification Service
• A universal Apple Push Notification Service (APNS) provider in Python
• Apple Push Notification Library for Haskell
• Apple Push Notification Library for Ruby
• Rails plugin for Apple Push Notifications
• Apple Push Notifications on Rails
• Apple Push Notification & Feedback Services Client C# Library
• Persistent Apple APNS interface for Perl 5

I will try to keep this list updated.

Edit: added ApnsPHP & pyapns.org
Edit: separated the active services from the beta, added msgpush.com and C# Library
Edit: added a Rails plugin
Edit: added AppNotify (in Beta)
Edit: AppNotify is no more in Beta.
Edit: Added EasyAPNS source code
Edit: Removed Bigcurl HTTPush, since it’s reserved for bigcurl agency customers
Edit: added a Perl module