Apple today added a step by step guide for setting up iPad 3G on AT&T in the US. Fun stuff pasted below.
iPad Wi-Fi + 3G allows you to sign up for a Cellular Data account directly on the device.
Follow these steps to set up your new Cellular Data account:
1. Tap Settings > Cellular Data > View Account:
2. The Cellular Data Account management window loads. Enter information or select an option in the following sections and tap Next when you are finished:
Enter your first and last name and your phone number.
Enter an email address and a password. This creates an AT&T account, allowing you to log in to the account later to make changes or view your usage statistics.
Recurring Domestic Plan Options
This allows you to choose the type of plan you want to use.
Payment & Billing Information
Complete this section with the billing information you