Next, you hacktivate your iPhone. iLiberty+ or any of the other tools out there allow you to stick in an AT&T SIM and activate the phone. This is not unlocking; this is adding authorization files directly to the phone. Your iPhone's modem remains unaffected and pristine.
Finally, you call 611 and enter the new phone number and your PIN. Tell the mechanical lady that you want to "Buy Features" and when she asks, "Data Packages". Choose the Unlimited MediaNet option. This costs $19.95 against that already-purchased $100 amount, and you're good for an entire month of unlimited data use.
What are the drawbacks? First, you *must* watch your dates and times carefully. When you go past your 30 days, you start paying at the outrageous standard data rate of $10/MB. Read that again. If you use the iPhone without the data-plan functioning, you might spend $20 to view a 2MB picture of a dog -- the way I did back in September.
Second, PAYG plans are not sanctioned by Apple. That means your non-AppleCare warranty coverage is practically non-existent. Caveat Emptor. It's not much worse than buying an iPod touch, but you will be SOL if you need any kind of Apple support.
Finally, if you plan to use data every single month, you'll need to keep adding money to your account every 5 months. The $100/one year arrangement only works until the money runs out. Make sure you're ready to re-fund that account as needed.
If you're willing to put up with all these negatives, you end up with a contract-free unlimited-data iPhone with the option of making calls for just $20/month or so. It's an amazing deal. You don't need a doctor's note. It works for both the hearing and hearing impaired alike. Just keep careful track of the plan and you're good to go.