There is not a difference in the SIM, almost any SIM within the last 36 months will work just fine. There is not a special iPhone SIM or a special 3G SIM. 🙂
The original iPhone pairs with the SIM during the iTunes activation. That's why her SIM works just fine in your iPhone 3G but your SIM does not work in the original iPhone without completing the iTunes activation to pair the iPhone with the SIM.
Dave
To be clear, the 3G standard did specify some additional modules to be included on what is commonly referred to as a "3G SIM card", which were not present in previous generations of GSM SIM cards. As such, previous generations of SIM cards would NOT connect to 3G networks. "3G SIM cards" are generally backwards compatible, and provide full functionality with, 2G phones and 2G networks.
(Historical note: in the 2G GSM specification, the concept of the physical card and the GSM SIM application running on it were inexorably bound together. In 3G networks, the card itself, technically called a UICC, Universal Integrated Circuit Card, may run several different application modules, including a SIM module to provide access to 2G networks, and a USIM module to provide access to UTMS networks. In theory, a "3G SIM card" could also contain a CSIM module to provide connectivity to traditional CDMA networks as well, but most exclusive GSM and UTMS providers wouldn't bother including such modules on their cards, and most phones capable of connecting to traditional CDMA networks either don't have any hardware present to accept external subscriber identity modules, or else they've been locked down by their carriers to ignore any external modules even if you inserted them.)
That being said, after AT&T started rolling out "3G SIM cards", they stopped issuing the older 2G SIM cards, so any relatively recent card issued by AT&T is likely a UICC containing both a GSM SIM module for 2G connectivity and a UTMS USIM module for 3G connectivity.
