I ordered an iPhone 5s via the Apple Store app on iPhone, signed up for their unlimited voice offering at $39.99/mo. to go along with my grandfathered unlimited data. AT&T now claims it was an error and no such plan exists! What a joke, they are still investigating, as is Apple. Order was placed on launch day, still not resolved.
For a large majority of customers, T-Mobile in the US is the better deal if you are on an AT&T/GSM iPhone today.