On the face of it, the micro-SIM used for both the iPad and the iPhone 4 is the same. So logic would dictate that you could buy yourself an iPhone, then pick up an iPad, using the same micro-SIM on both devices, saving a few quid in the process. Sadly, that's not how Apple itself sees it.The question and answer on the Apple FAQ site goes like this:Are iPhone 4 and iPad micro-SIM cards interchangeable?No. Your iPad micro-SIM card will not work with your iPhone.End of discussion. Well, almost. According to V3, a spokesman for O2, one of the companies in the firing line, has agreed to put a little meat on the bones of Apple's rather pointed answer. Nick Wilkins, press relations manager at O2, said that an iPhone 4 micro-SIM inserted into an iPad will detect that it is not in a smartphone and will not work.He added: 'The micro-SIMs for the iPhone are set up to allow voice calls, SMS messages and data functionality, whereas the iPad micro-SIM is provisioned to allow pay-as-you-go data transfer only. The two devices are designed to give a different experience. The iPhone 4 is for regular use, whereas with the iPad we offer more flexibility over data use.'That makes sense if you are trying to use a micro-SIM from an iPad into an iPhone 4, but if you have sufficient data allowance on your iPhone 4 deal, surely there isn't a problem with slipping that into your iPad? Apparently there is - the two cards are, at the moment, mutually exclusive. This, despite the European Telecommunications Standards Institute, which oversees SIM cards, claiming there are three recognised sizes - full, mini and micro - but no difference in the circuitry.A technical issue or company policy? This one is likely to run and run.