Calling AT&T
As June 29th approaches, it's probably best if you called around to your local Cingular/AT&T vendors in advance to make sure they'll actually be carrying the iPhone. According to ZDNet, only corporate AT&T stores will carry the iPhone. Third party authorized franchises will not.
I've recently made a few calls of my own. I called AT&T to ask about their new on-the-go data plans. I was transferred to a data plan "expert". (I believe that is something like an Apple "genius".) She asked me which phone I'd be using and I said the iPhone. She asked me what that was and who sold that.
I also called my local Cingular (now AT&T) store. They quickly confirmed that they would be carrying the iPhone and that it would be available on June 29th, but then continued on without my prompting, telling me that they knew pretty much nothing more than that including whether the phone would be on sale in the morning or at 6PM and that they did not yet know what it would cost and had no further details to give. One suspects I was not the first to call. At least they were forthcoming and clueful.