MicroSIMs come with a couple extra features:

• allow for multiple simultaneous applications accessing the card through logical channels;
• introduce mutual authentication as a way to eliminate carrier spoofing by allowing the SIM card to authenticate the cell tower to which it is connecting;
• add a new PIN protection with hierarchical PIN management with a universal PIN, an application PIN and a local PIN;
• expand the phonebook storage of the SIM card with entries for email, second name, and groups.

Why does the iPhone HD use a MicroSIM just like iPad|The iPhone User Guide

forgot, hardware locking the device to AT&T while saying it is an unlocked device.