According to T3, the iPhone 4 micro SIM is “not compatible” with iPad, dashing the hopes of those who want swap SIMs around:
“The plan, had this not been the case, would have been to buy a 3G iPad but not pay for any data, then simply insert your iPhone SIM into the tablet to leech off its 3G network capabilities under your mobile contract’s data allowance. The official line on this form Apple goes: ‘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.’”
(It’s worth noting that the link that T3 provides to Apple’s FAQs leads to a page which doesn’t say this).
This didn’t quite smell right to me. So I asked around, and the consensus amongst phone network folk is that it’s very unlikely that there’s anything preventing you putting an iPhone 4 SIM into an iPad and it just working.
Networks can apply limitations based on IMEI number, which is tied to the hardware, so a network could block an individual iPad or iPhone. A network could, in theory, pair an IMEI to SIM ID on its own systems when you first use your iPhone, and prevent any other IMEI from working with the same SIM – but this would be a pain to provision just to prevent a tiny number of people swapping SIMs around. It just wouldn’t be worth the hassle.
There is, though, one possibility: SIMs do provide a user-writable area, so Apple could write a flag there when a SIM is first used in an iPhone which the iPad checks for. If the flag is there, the iPad would then refuse to work. But this would be unlikely, tricky to do, and rather pointless from Apple’s perspective – after all, some networks might actually want to sell a “one SIM, two devices” option in the future.
My gut feeling is that T3 has just taken what Apple has written and pushed the story too far. It’s not a compatibility issue, in the sense of “incompatible SIMs” – it’s a provisioning issue, in the sense of “your carrier may well be pissed off when they notice how much data you’re using”. No doubt someone will try it out in a few weeks and we can see!