I have downloaded the Amazon Appstore apk through the web browser on my Aria over 3G and it worked fine. I just now tried it again and it still works. AT&T might block certain types of traffic, but they don't do anything to prevent you from downloading the Amazon Appstore apk. (They do however prevent you from installing the apk once it's on your phone because they removed the ability to install applications from "unknown sources", but this restriction is removed in all of the custom ROMs on this site.)
That said, I have had problems downloading files through the web browser before -- it's quite random though. I believe this is actually a Google bug, not an AT&T bug, but I'm not sure.
That said, I have had problems downloading files through the web browser before -- it's quite random though. I believe this is actually a Google bug, not an AT&T bug, but I'm not sure.