Yesterday we talked with AT&T about the carrier's new pricing olive branch they extended to current iPhone customers eager to upgrade to the 3GS at subsidized prices. At first glance, it appeared that AT&T was just giving customers whose contracts were up within the next three months a three month reprieve -- allowing them to upgrade to the 3GS at the full subsidized price point of $199 or $299. But there's a little more to it then that.

The first thing you need to understand is that not all iPhone users are created equal when it comes to their upgrade eligibility window. Some customers who signed a two-year contract to get an iPhone 3G are upgrade eligible after just a year. Different users have different upgrade windows based on a number of factors, which AT&T isn't completely sharing. One of those factors is whether you spend $99 a month or more.

"In terms of exactly what goes into the eligibility calculation, for competitive reasons, we have not published all the details of our policy," AT&T's Seth Bloom tells us. "But it is based on monthly recurring charges, on time payments/account in good standing, and other factors," he says. "As you’d expect, in general, the more you spend with us, the less time you have to wait to purchase another device at a discounted price."

Yesterday's announcement (also see FAQ) (pdf) by and large helps those customers that are under AT&T's good graces. We asked AT&T for a concrete example, just for clarity's sake.

"Let’s say you have an upgrade date that’s 14 months into your 2 year contract -- a pretty good thing by itself, since you signed a 2-year contract," says Bloom. "If you bought iPhone 3G in July of last year, that date would be around September 2009," he says. "Until today’s announcement, that means that if you wanted to wait for the full subsidy, you’d need to wait to buy a 3G S in September," says Bloom.