When purchasing an iPhone 3G, you'll need a voice plan, a data plan, and a text messaging plan in order to use the phone. Since the iPhone 3G only works on AT&T's network, your choices are limited to the plans that AT&T offers.
Here's a detailed list of the iPhone 3G plans available from AT&T.
Unlimited Data Plan: $30 per month
This is a requirement; AT&T does not offer another data plan for the iPhone 3G.
AT&T offers a choice of voice plans for the iPhone 3G. All offer a certain number of nationwide calling minutes, and all offer free calling to other AT&T mobile phones.
All of the plans (except the unlimited plan) include rollover minutes, which allow you to save unused voice minutes and apply them to your next bill should you go over your allotment. Additional minutes cost between 35 cents and 45 cents per minute, depending on your plan.
Most of the plans offer unlimited free night (9pm to 6am) and weekend calling; the only one that doesn't is the plan with 450 minutes, which limits you to 5000 night and weekend minutes.
All of the plans also require that you pay a one-time activation fee of $36 to set up the line.
The first-generation iPhone did not require you to pay for a text plans; all text messages sent and received were included as part of the unlimited data plan. That's no longer the case. If you plan on using your iPhone 3G to send text messages, you'll want a text plan. Here are the options.
Without a plan, you'll be charged 20 cents for each text message you send or receive.