I need to add an in-app purchase option to my iPhone application. All it needs to do is to unlock the additional content - the content is there, in the bundle alredy, it's just a matter of setting a flag to premium - yes or no.
As it's fairly easy to implement (I just need to check if appstore transaction succeeded or failed and set the flag accordingly), I'm wondering what would happen if user deleted the app and then reinstalled it - all data stored on the device will get deleted when app is removed...
What is the correct way to perform such purchase? Or will the appstore somehow notify the app of the purchases made from this account when the app is re-installed?
It depends. If the premium content is non-consumable, the appstore will keep track of the users purchases. For consumable content however, you're required to keep track yourself using your own server infrastructure.
See "Restoring Transactions" of the In App Purchase Programming Guide.