So, I understand that it's possible to set up in-app purchases for iPhone apps to purchase non-consumables like game levels. I understand the logic behind the purchase part, but what I don't understand is, how can I deliver the new game level.

For example: I build an app that contains the first level and they purchase additional levels. Is it better to build all the other levels into the app and whenever they purchase the app, it unlocks it with a plist entry or something? That doesn't seem very update-able to me. Every time I come up with a new level, I'd have to update the app.

So, what I don't understand then, is what is how do I package up a level and download it as a separate entity that can accessed by the game? Would the level just be some XML with images in a ZIP folder or something? How does the level get added to the game? What are best practices for this type of thing?

I Googled and have found NOTHING about this. I'm a little bit confused by the concept and any help would be appreciated. I'm not looking for someone to write the game for me, I just need pointed in the right direction so I can develop it on my own.