So can I just pop my iPhone SIM (with the iPhone unlimited data plan) into __________ and access data?
Can I Use The SIM card from my iPhone in a SONY Ericsson PCMCIA or ExpressCard modem or Nova Media Global Surfer ICON USB Modem?
The answer is no. If you try, at first it may look like the modem will connect, but as soon as you try to access data the modem will disconnect.
Why is this?
Your iPhone has a "network serial number/authentication" called an IMEI number (IMEI = International Mobile Equipment Identity). This number is specifically tied to an iPhone account and was first registered on the AT&T network when you first activated your iPhone.
What if I have a hacked / unlocked iPhone and I'm using it on another network?
It still won't work at least in the USA. Your IMEI number does not change. AT&T is the predominant carrier for GSM in the United States. Most of the time, you will be roaming on the AT&T network EVEN IF you have a different carrier - as T-Mobile and Suncom lease a lot of their network from AT&T.
To get a data plan with other devices (including modems for laptops) one must purchase a separate data plan from AT&T or their cell phone provider.