I really think it's an effort to squeeze out a little more life from the old and busted. If you can maximize your earnings without a large capital outlay, you make your investors very happy. AT&T has the opportunity to upgrade an existing and outdated network with a minimal (comparatively) investment, tying the hottest phone ever created since Mr. Bell was still kicking to it and reaping a HUGE profit margin. The alternative would be to invest significant revenues into the 3G network, which would also have to be updated to handle the enormous increase inusers and traffic. In my uninformed guess, I expect that it's cheaper to upgrade EDGE than to upgrade 3G. So, for at least the first big round of phones, AT&T gets away with using the slower network and making a large profit. Think 802.11b vs. 802.11n in costs off the shelf.
All that said, I bet that the next generation phone has 3G and will give AT&T the time to plan the upgrade of that network in the interim. But, like I said in my previous post, I bet there is some agreement in place that Apple will refrain from replacing this first generation until AT&T has had ample time to actually realize the return on their investment.
ilLegalBeagle