Currently only AT&T and Apple have been announced as selling the iPhone. NO ONE ELSE. that includes Walmart, Radio Shack and Best Buy. Granted they probably will sell them eventually but nothing has been even hinted at that they will be able to sell them.
My company buys computers from PC Connection. They are an authorized Apple reseller. They can sell _anything_ the Apple store can sell. I called my sales rep over there and asked about the iPhone and she said no. I just sent her this Amazon link and she was surprised. She is going to ask her Apple rep about it so we will see what they say.
I just find this link on Amazon very very surpriseing.