I may have misinterpreted. Another blogs says: "up to 6 months of early eligibility".
That may mean shortening a 24-month contract to 18 months. However, AT&T already allows you to upgrade after 21 months.
It used to be, and I believe still is, the case that if you spend $99 or more a month, you’re eligible for the upgrade sooner. The AT&T announcement, at least as I read it, is that you’re eligible for the subsidy 6 months earlier than you would be otherwise. The contract, however, is still 24 months, as you’ll find out if you try to cancel service.
Just to engage in wild and irresponsible speculation, I’m guessing AT&T knows that it’s exclusivity deal is up in the next year at longest. They’re bumping up the eligibility date to lock in as many folks as possible for two years before other carriers jump into the iPhone game.