How to create a GUID/UUID using iOS

[[UIDevice currentDevice] uniqueIdentifier]

Returns the Unique ID of your iPhone.

EDIT: -[UIDevice uniqueIdentifier] is now deprecated and apps are being rejected from the App Store for using it. The method below is now the preferred approach.

If you need to create several UUID, just use this method (with ARC):

+ (NSString *)GetUUID
{
  CFUUIDRef theUUID = CFUUIDCreate(NULL);
  CFStringRef string = CFUUIDCreateString(NULL, theUUID);
  CFRelease(theUUID);
  return (__bridge NSString *)string;
}

EDIT: Jan, 29 2014: If you're targeting iOS 6 or later, you can now use the much simpler method:

NSString *UUID = [[NSUUID UUID] UUIDString];

Here is the simple code I am using, compliant with ARC.

+(NSString *)getUUID
{
    CFUUIDRef newUniqueId = CFUUIDCreate(kCFAllocatorDefault);
    NSString * uuidString = (__bridge_transfer NSString*)CFUUIDCreateString(kCFAllocatorDefault, newUniqueId);
    CFRelease(newUniqueId);

    return uuidString;
}

In iOS 6 you can easily use:

NSUUID  *UUID = [NSUUID UUID];
NSString* stringUUID = [UUID UUIDString];

More details in Apple's Documentations

Reviewing the Apple Developer documentation I found the CFUUID object is available on the iPhone OS 2.0 and later.

In Swift:

var uuid: String = NSUUID().UUIDString
println("uuid: \(uuid)")

The simplest technique is to use NSString *uuid = [[NSProcessInfo processInfo] globallyUniqueString]. See the NSProcessInfo class reference.

In Swift 3.0

var uuid = UUID().uuidString

I've uploaded my simple but fast implementation of a Guid class for ObjC here: obj-c GUID

Guid* guid = [Guid randomGuid];
NSLog("%@", guid.description);

It can parse to and from various string formats as well.