Please note that I currently use AT&T Wireless. There are two drawbacks to this deal, or more like two things to comment on.
Number one, AT&T currently has a $50 credit program that can be applied to any phone. This means that the Sony Ericsson T616 phone and others are free directly through AT&T. Although you may make money on Amazon, buying through the carrier direct gives you better support and better warranty options.
Number two, when you activate a plan through Amazon or other resellers, you miss out on any AT&T deals that they are giving direct. Right now, for example, if you sign up for their $40/month mLife national plan which includes 550 minutes, you get an extra 50 minutes free each month, permanently. Therefore, it is 600 minutes for $40/month. Amazon does not offer these offers, and many more.
Buying through the provider direct is always more wise, so just take these deals with a grain of salt.