Every single iPhone and iPad transmits its hardware serial number to Apple when you launch the App Store app, or on first boot after restore for "activation".
> Intel has revealed that each Pentium III chip will carry a unique serial number that can be read by the computer's software. Intel claims that the serial number will facilitate e-commerce, promote "digital content protection," prevent counterfeiting of Intel processors, and help to track stolen ones. We know users have questions about this controversial feature, so we assembled this FAQ. Q: Why are privacy experts concerned? A: Privacy experts are concerned because the CPU's electronic serial number could be used for purposes that may not be in users' best interests.
Anyway, speaking of unique identifiers in mobile devices, mobile phones have had IMEIs for ages - pre-dates Apple by a long time.