The iPhone Blog posted a message about a possible payment when you want to re-download an app. Right now you can re-download the app as many times as you want without re-paying for it.
The picture shown below shows the pop-up box with the that message but also points out that users may still be able to re-download the app for free via their computer.
The reason could be to eliminate the use of several devices with one account. When one person downloads and pays the app via, let’s say the account name is “Steve Jobs”, then a second person who also wants to download that app via the same account has to repay for it since it already is in possession of the first person.
Maybe this system can recognize a unique code specific for that device, so it can ’see’ when a second device (with a different unique code) wants to download it through the same account.
We’ll see if this is implemented in the new OS 3.0 software. Would you be happy with a system like this?

If you get a new iPhone, because your old is damaged, then the unique code would be different. You would have to pay again for something that you genuinely own! In that case Apple would have to transfer the unique code from one iPhone to another one, which opens again the field of fraud.