NSString+UUID.h

#import <Foundation/Foundation.h>

@interface NSString (UUID)

+ (NSString*)stringWithUUID;

@end

NSString+UUID.m

#import "NSString+UUID.h"

@implementation NSString (UUID)

+ (NSString*)stringWithUUID
{
    CFUUIDRef uuid = CFUUIDCreate(kCFAllocatorDefault);
    NSString * string = (NSString*)CFMakeCollectable(CFUUIDCreateString(kCFAllocatorDefault, uuid));
    CFRelease(uuid);
    return [string autorelease];    
}

@end

Simple category for adding a method to NSString for creating UUID's

For an ARC compatible implementation: https://gist.github.com/1501325

Although this is a method, not an extension to NSString.