What is Micro SIM? How is it different from a regular SIM card? Does the iPad have a SIM card?

In a nutshell, Micro SIM -- which is the SIM (Subscriber Identity Module) standard used by the high-end US$629-US$829 configurations of the original iPad and the GSM-compatible version of the iPad 2 -- is a smaller version of the Mini SIM cards commonly used and that frequently are referred to merely as SIM cards.

Photo Credit: Wikipedia

As first noted by PCMag, Micro SIM is the third version, or "third form factor" (3FF) of the SIM standard and holds the same amount of data:

3FF SIMs were developed by the European Telecommunications Standards Institute to fit into devices too small for a regular [Mini] SIM. Where a regular SIM measures 15 millimeters x 25 mm, a 3FF SIM card measures 12 mm x 15 mm. . .

Today's "regular" sim is actually the second SIM form factor, shrunken down from the SIM card's original credit-card size.

The Register explains further:

Apple has been saying the Micro SIM will also support a more-comprehensive phone book as well as greater security. In fact, both of these capabilities are part of the USIM [Universal Subscriber Identity Module] application, which can be installed on a SIM of any size or shape as part of the 3G standard.

The traditional SIM address book supports only name-number pairs, but the USIM requires support for multiple phone numbers attached to a single name, as well as a field for an e-mail address and user-definable groups.

C|Net confirms that the size is the essential difference between the Micro SIM and the earlier Mini SIM standard:

The European Telecommunications Standards Institute (ETSI), which sets the standards for SIM cards, tells us that except for the size of the plastic around the chip, micro SIM cards are identical to the SIM cards we normally find in phones.

That means in theory, nothing is stopping us from trimming down a standard SIM card and popping it in the iPad.

Sure enough, a "trimmed" SIM card does work in the iPad models. As first spotted by Gizmodo, shortly after the original iPad shipped, a hardworking hacker shaved a standard SIM card down to fit and hacked the iPad to send text messages as well.