Time was a developer could call the UDID (unique device identifier) for your iPhone, iPad, and iPod touch and do with it what they would, from beneficial things like keeping your data allocated to you, to evil things like tracking you and your behavior for advertising and giggles. Now with iOS 5 beta 6 Apple aims to make it no longer so.
Deprecated in iOS 5.0
An alphanumeric string unique to each device based on various hardware details. (read-only) (Deprecated in iOS 5.0. Instead, create a unique identifier specific to your app.)