I don't know, I'm gonna go ahead and disagree with you there. Although I don't have any definitive proof, all evidence leads me to believe that AT&T is what is causing the problem.
I do think it was AT&T blocking the application (amazon appstore) from downloading. Traffic shaping rules in the U.S. do currently allow wireless carriers to block specific protocols and data from being transferred over their networks. For example, bit-torrent traffic and specific text messages can be blocked on the premise that it is for the common good. The text message blocking part is particularly nasty in my eyes, but it is allowed because there is the potential that carriers can block unwanted spam texts from propagating through their networks.
I recall similar things being discussed 6 months ago for otherr applications. The applications in question would not download if you were on the AT&T network and trying to download via 3G. I had to switch over to wifi in order for the download to complete successfully. And yes, 3G was working perfectly fine, I downloaded some other applications via marketplace just to make sure.
Amazon even wrote in their FAQ that you cannot download amazon appstore if you're on the AT&T network, so I think that pretty clearly means that my assumptions were correct. You may say that their comment was directed at non-rooted phones but that too is not true. Their FAQ lists being rooted as one of the prerequisites for installing amazon appstore.
Unless, drumist, you think the application just doesn't like to download via 3G for anyone..?
This post sounded harsher than I ment for it to. I'm not really sure why it turned out that way.
