Their press release
indicates that it will be like a toll-free phone call, or like free shipping, but it will likely be more like advertising supported content.
"With the new Sponsored Data service, data charges resulting from eligible uses will be billed directly to the sponsoring company; the customer simply enjoys their content via AT&T’s wireless data network. Customers will see the service offered as AT&T Sponsored Data, and the usage will appear on their monthly invoice as Sponsored Data. Sponsored Data will be delivered at the same speed and performance as any non-Sponsored Data content."
From the FAQ
"Q: What is AT&T Sponsored Data?
A: Sponsored Data is an AT&T product that enables companies to sponsor the data usage for specific content on behalf of eligible AT&T wireless customers. With AT&T Sponsored Data customers can browse, stream and enjoy applications from our data sponsors without impacting their monthly data plan allowance."
Of course, it's not for everyone:
"Sponsored Data is available to post-paid and session-based customers who have an active AT&T data plan, a 4G (HSPA+ and LTE) capable device - including smartphones, tablets, laptop modems, and mobile hotspots - and access the Sponsored Data over AT&T's domestic wireless network. Other limitations and may restrictions apply."