UIDevice uniqueIdentifier deprecated - What to do now?

A UUID created by CFUUIDCreate is unique if a user uninstalls and re-installs the app: you will get a new one each time.

But you might want it to be not unique, i. e. it should stay the same when the user uninstalls and re-installs the app. This requires a bit of effort, since the most reliable per-device-identifier seems to be the MAC address. You could query the MAC and use that as UUID.

Edit: One needs to always query the MAC of the same interface, of course. I guess the best bet is with en0. The MAC is always present, even if the interface has no IP/is down.

Edit 2: As was pointed out by others, the preferred solution since iOS 6 is -[UIDevice identifierForVendor]. In most cases, you should be able use it as a drop-in replacement to the old -[UIDevice uniqueIdentifier] (but a UUID that is created when the app starts for the first time is what Apple seems to want you to use).

Edit 3: So this major point doesn't get lost in the comment noise: do not use the MAC as UUID, create a hash using the MAC. That hash will always create the same result every time, even across reinstalls and apps (if the hashing is done in the same way). Anyways, nowadays (2013) this isn't necessary any more except if you need a "stable" device identifier on iOS < 6.0.

Edit 4: In iOS 7, Apple now always returns a fixed value when querying the MAC to specifically thwart the MAC as base for an ID scheme. So you now really should use -[UIDevice identifierForVendor] or create a per-install UUID.

check this out,

we can use Keychain instead of NSUserDefaults class, to store UUID created by CFUUIDCreate.

with this way we could avoid for UUID recreation with reinstallation, and obtain always same UUID for same application even user uninstall and reinstall again.

UUID will recreated just when device reset by user.

I tried this method with SFHFKeychainUtils and it's works like a charm.

Based on the link proposed by @moonlight, i did several tests and it seems to be the best solution. As @DarkDust says the method goes to check en0 which is always available.
There are 2 options:
uniqueDeviceIdentifier (MD5 of MAC+CFBundleIdentifier)
and uniqueGlobalDeviceIdentifier(MD5 of the MAC), these always returns the same values.
Below the tests i've done (with the real device):

#import "UIDevice+IdentifierAddition.h"

NSLog(@"%@",[[UIDevice currentDevice] uniqueDeviceIdentifier]);
NSLog(@"%@",[[UIDevice currentDevice] uniqueGlobalDeviceIdentifier]);

XXXX21f1f19edff198e2a2356bf4XXXX - (WIFI)UDID
XXXX7dc3c577446a2bcbd77935bdXXXX - (WIFI)GlobalAppUDID

XXXX21f1f19edff198e2a2356bf4XXXX - (3G)UDID
XXXX7dc3c577446a2bcbd77935bdXXXX - (3G)GlobalAppUDID

XXXX21f1f19edff198e2a2356bf4XXXX - (GPRS)UDID
XXXX7dc3c577446a2bcbd77935bdXXXX - (GPRS)GlobalAppUDID

XXXX21f1f19edff198e2a2356bf4XXXX - (AirPlane mode)UDID
XXXX7dc3c577446a2bcbd77935bdXXXX - (AirPlane mode)GlobalAppUDID

XXXX21f1f19edff198e2a2356bf4XXXX - (Wi-Fi)after removing and reinstalling the app XXXX7dc3c577446a2bcbd77935bdXXXX (Wi-Fi) after removing and installing the app

Hope it's useful.

EDIT:
As others pointed out, this solution in iOS 7 is no longer useful since uniqueIdentifier is no longer available and querying for MAC address now returns always 02:00:00:00:00:00

You can use your alternative for Apple UDID already. Kind guy gekitz wrote category on UIDevice which will generate some kind of UDID based on device mac-address and bundle identifier.

You can find code on github