There's quite a message thread about this on Slickdeals. It looks as if the deal may be gone after quite a few people seem to have exploited it. You did have to add M2M (month to month) to an existing AT&T line however there was no contract. So lots of people on the deal sites were saying they ordered the phone(s) and as soon as they received shipping notices they cancelled without any penalty.
Last I read the deal was no longer active for M2M though I think you can get it for 2 year contract phones which require you to add a line. I don't know if it's still working for upgrades.
YMMV
http://slickdeals.net/permadeal/97508/att-wireless-att-50-off-smartphones-w-2year-contract-16gb-samsung-galaxy-s4-100-32gb-htc-one-smartphone-100-16gb-apple-iphone-5?#comments