The decision to use an unusually small SIM card in the iPad means current iPhone customers can't just use their iPhone's SIM in an iPad, as we reported last week.

It appears increasingly likely that this decision was driven not by space considerations (after all, the iPad is much larger than the iPhone), but rather by AT&T's business strategy. This way, they can charge customers twice for 3G wireless data: Once for the iPhone, and once for the iPad.

Gemalto, which probably makes the micro SIM card in Apple's iPad, says OEMs might choose to use a micro SIM card because mobile operators set device requirements forcing them to do so. Gemalto couldn't confirm that it was an Apple supplier, but according to Lok8u, the manufacturer of the only other micro SIM card device currently sold in America, Gemalto is the likely maker of the iPad's SIM.

"We cannot speak specifically about our relationship with OEMs," said Gemalto North America vice president of business development Jean-Louis Carrara.

But he was able to speak about the company's micro SIM card clients in general, shedding light on Apple's strange decision to use a smaller card in the large iPad device and a larger card in the small iPhone.

The main reason a manufacturer would choose micro SIM, also called 3FF, is to facilitate smaller cellphone designs, as has happened already in Europe, said Carrara.

However, he acknowledged that purely business reasons could play a role in a company's decision to go with a format that's all-but-unknown in the United States.

"If you look at where the micro SIM is being used today, it's in those devices that have real space constraints, but it's hard to envision a tablet to have that much of a constraint," said Carrara. "Maybe it does. But it could also be a decision by some OEMs that they would prefer to have a [micro SIM] because it just makes sense with the way they want to run their business, or the customer segment, or the way they want to distribute their devices. We will see OEMs choosing [micro SIM] because they have a physical constraint, but it could also be because they want to have a card that is different than the plug-in SIM."

No company that works with Apple can say much about it, and Apple and AT&T are themselves notoriously tight-lipped. There is no evidence that the smaller SIM was chosen as a concession to AT&T. But the circumstantial evidence suggests that it was not chosen because of the limited space in the iPad's chassis.

Carrara agreed with our prediction that a certain segment of buyers will rebel against the purchase of a second wireless data plan by slicing their SIM cards so that they work both in their phones and the iPad.

"There is no difference between the functionality or design of a micro SIM card, when compared to a regular SIM card – the only difference is the size of the extra plastic surrounding the chip," confirmed Carrara. "Cutting the plastic around [a SIM card] and putting it in a new device is not endangering anything. What it's probably going to do is that people will probably not cut the plastic properly, and the contacts won't match on the device, and therefore it won't perform, but that's not a problem. You can lose your SIM today and go to the operator and get a new one."

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