Mine

io_service_t platformExpert = IOServiceGetMatchingService(kIOMasterPortDefault, IOServiceMatching("IOPlatformExpertDevice"));

if (platformExpert) {
	CFTypeRef serialNumberAsCFString = IORegistryEntryCreateCFProperty(platformExpert, CFSTR(kIOPlatformSerialNumberKey), kCFAllocatorDefault, 0);
	if (serialNumberAsCFString) {
		NSLog(@"SN: %@",serialNumberAsCFString);
	}
	IOObjectRelease(platformExpert);
}