AT&T will be discontinuing their unlimited data plans come July 7th. Anyone who signs up for an unlimited plan before July 7th will be allowed to keep their level of service afterwards. Currently, for $30/month prepaid, iPad 3G users can get unlimited access. Those without iPads are normally required to purchase more expensive data plans on a two year contract. Nothing is stopping someone with an iPad plan from moving their SIM to another device, but they had to purchase an iPad 3G for the privilege.
AT&T offers replacement iPad SIMs on their website for $15:
As pictured, the card I received was scored twice to allow the user to decide if they want to use it as a mini or micro SIM. Anyone can purchase one, but needs a friend with an iPad 3G to activate it.
For those without friends or the nerve to go into an Apple Store to try and activate their card covertly, they can still activate their card using this URL:
replacing Y with the ICCID of their SIM and replacing X with any valid IMEI. The service isn't locked to iPad IMEIs, and even "0" works as a valid entry, but to be safe one might want to first activate using the IMEI of their device.
This website, however, is only accessible if the user's browser reports itself as an iPad.
The best solution, for those with Macs, is to just use the iPad simulator included with the iPhone SDK.
Desktop Safari can work if you enable the Develop menu and change the user agent to something like "Mozilla/5.0(iPad)", but the site uses a lot of touch functions that only work on Mobile Safari, so while one might be able to sign up for service, they won't be able to change their account settings later. A workaround is to use "Disable Styles" under the Develop menu which will strip the site down to the bare usable minimum. Other browsers render the site poorly, and aren't recommended.
Once someone accesses the site, they can register for service just like on the iPad. When they return to the site with the same ICCID in the URL (IMEI, again, can be anything valid or "0") they'll be asked for the email address and password used to create the account. They can now see how much data and time is left in the month and change billing settings.
After service is purchased, the SIM can be used on any GSM device using the APN "broadband"
If one is without internet access and only has an unactivated SIM, they can use the APN "LWAActivate". This grants walled access to the activation server. Interestingly, full DNS access is available like on most captive portals and DNS tunneling works, albeit very slowly.