Unlike the old iPhone, the iPhone 3G works without a contract for both voice and data, completely unmodified. No jailbreaking or unlocking required: use any AT&T Pay As You Go SIM card with a data plan and it will work. The process is quite simple, just replace the SIM card with the Pay As You Go card and you'll be doing calls and accessing the internet instantly. The question is: Why?
Would this work for international trips with other Pay As You Go cards? That's the only scenario that seems logical to me, because at this point, you can't buy an iPhone 3G without a contract with AT&T. Being able to use a Pay As You Go card defeats the purpose, as far as I can see. Since you have to pay the monthly rate anyway, why use Pay As You Go?
In any case, if you can think into any useful thing to do with this, Erica at TUAW details how to attach an unlimited 5GB plan to a SIM:
• Call 611
• Select "Buy Features"
• Select "Media Net"
• Select "Unlimited"
• Select "Buy It"