AT&T (NYSE: T) has just announced that it will be waiving the additional upgrade fees for some subscribers, who have been very vocal about having to pay hundreds of dollars to upgrade to the iPhone 3G S when they are just weeks away from being eligible for a new device. Release.

In preparation for the phone’s launch on Friday, AT&T wrote today: “We’ve been listening to our customers. And since many of our iPhone 3G customers are early adopters and literally weeks shy of being upgrade eligible due to iPhone 3G S launching 11 months after iPhone 3G, we’re extending the window of upgrade eligibility for a limited time.”

AT&T said starting tomorrow, iPhone 3G customers who are eligible for an upgrade in July, August or September, will get the best price available ($199 for the 16GB device). If you have already ordered the device, you will receive a refund. If you still don’t fall within this three-month window, AT&T said you can pay the “early upgrade price” of $399 (16GB) or $499 (32GB), or pay full retail — and not have to sign a 2-year contract — at $599 (16GB) or $699 (32GB).

As part of this announcement, AT&T did a good job providing some rhyme or reason as to how they price new phones and determine when a subscriber is eligible for a new one. It wrote that the more a customer spends every month, the quicker they become eligible for a price break on a new device. “For example, iPhone customers who spend more than $99 a month per line with us generally are eligible for an upgrade between 12 and 18 months into their contract.”

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