Moving from regular SIMs to micro SIMs for the iPad and iPhone 4 was a fairly user-hostile move on Apple's part, seeing how it made moving between an iPhone and virtually any other GSM device quite a bit more difficult; the official explanation (for the iPhone 4, anyway) was that there simply wasn't room for a regular SIM, but that seems unlikely considering that you're only saving 10mm of length, 3mm of width, and 0mm of thickness. A more plausible scenario is that Apple viewed micro SIMs as a way to get ahead of the industry curve a bit and make it unpleasant for users to try to share an iPhone line with other devices, a concept that gets at the core of Apple's mantra of owning the entire
