If you use cookie tracking in your apps for iOS be warned, Apple might reject your next update! Basically - if you use Safari in anyway (uiwebview or other web controllers) to initiate storing information into the browser cache (cookies) for advertising use or even app retrieval use (again using a web view) your app might be in danger of rejection. Alot of developers turned to this method when Apple decided to deprecate their UDID system.
The UDID is a unique 40-character string containing letters and numbers that is tied directly to the hardware signature of the device, it allowed developers and ad networks a way of collecting information about users. Though the UDID itself doesn’t contain personally identifiable information about the device’s owner, it can be used in combination with information collected by a third-party, including names, location, preferences, app usage and more, to paint a more complete picture of an individual.
Apple's answer to developers is their 'Advertising Identifier' system which is proprietary to iOS and allows users to disable all tracking from within an app - regardless of the method. Cookies was a way to thwart using their system but it's clear Apple is beginning to enforce it's use throughout their system.
There are other UDID alternatives developers are coming up with, including a few where you generate UDID's
from the MAC address of the device, but those too might be eventually phased out. In the end, it's up to Apple's discretion.