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Only recently, Apple has begun allowing its customers to sign up for an AT&T service plan without ever having to visit an Apple Store. (link)

With this online service you can do the following:

1. Enter your billing information.
2. Review eligibility and pricing.
3. Select your AT&T rate plan.

While completing this step certainly speeds up the process of buying an iPhone 3G, it does not eliminate the fact that you must still pay a visit to an Apple Store. 

According to Apple’s website, after completing the online phase, you must “choose your closest Apple Retail Store and let us know when you’d like to come in to complete your purchase. An Apple Specialist will be ready to help you personalize your new iPhone 3G, make calls, browse the web, receive email, and more.”

You can get started here

[MacRumors] [Apple]