What are accepted methods to reduce iPhone application piracy, which do not violate Apple's evaluation process?

If my application "phones home" to provide the unique device ID on which it runs, what other information would I need to collect (e.g., the Apple ID used to purchase the application) to create a valid registration token that authorizes use of the application? Likewise, what code would I use to access that extra data?

What seem to be the best available technical approaches to this problem, at the present time?

(Please refrain from non-programming answers about how piracy is inevitable, etc.)