Its important to note the difference between a UDID and a UUID.
UDID "unique device id" is hardware specific. It never changes for a particular device. For this reason, it has become a privacy concern and Apple is blocking apps that try to use this. As a result, Apple has generated an opt-out-able "device id" hash, particularly for advertisement usage. This new ID hash is called IFA and is available in iOS 6.0+.
UUID "universally unique id" is not hardware specific. It is a hash used to identify a device; but not particularly an absolute value. For example, PhoneGap generates a UUID based on device properties; this is what you get when you do device.uuid. If you delete the app and reinstall, you will get a new id hash. UUID is not being blocked by Apple.
I think the best solution in your case would be to use the IFA, with OpenUDID as a backup for iOS < 6.0.
Here is the code we use. If IFA is not available, get OpenUDID. [[You must install OpenUDID, read more about that here, https://github.com/ylechelle/OpenUDID.]]
NSString* uuid = nil;
if ([[UIDevice currentDevice] respondsToSelector:@selector(identifierForVendor)]) {
// IOS 6 new Unique Identifier implementation, IFA
uuid = [[[UIDevice currentDevice] identifierForVendor] UUIDString];
} else {
// Before iOS6 (or if IFA disabled) you shoud use a custom implementation for uuid
// Here I use OpenUDID (you have to import it into your project)
// https://github.com/ylechelle/OpenUDID
NSString* openUDID = [OpenUDID value];
uuid = [OpenUDID value];
}