AT&T is once again selling unlimited smartphone data, but you'll have to subscribe to either DirecTV or U-verse TV to get it. The wireless plan with unlimited data, voice calling, and text messaging costs $100 a month for one person and another $40 for each additional smartphone or tablet. The fourth line is free, so a family of four can get the unlimited plan for $180 a month (though they'd have to pay the full $220 for the first two months before the credit kicks in).

One bit of bad news: the unlimited plan doesn't include tethering, so you can't share the phone's Internet connection with another device.

The new deal that was just announced will be available beginning tomorrow for new and existing wireless customers who already have or add TV service. Customers who buy this bundle will also get a $10-per-month discount on their combined TV-and-wireless bill.

AT&T completed its acquisition of DirecTV in July and has been bundling wireless and TV services together in an attempt to boost both businesses. TV watchers who stream a lot on the go may be interested in the deal, as DirecTV makes 111 channels available for live streaming outside the home, with on-demand streaming content from another 94 networks, AT&T said. The unlimited mobile data can be used with any video or content regardless of the source, though.

AT&T says the unlimited data offer is a limited-time one, but it didn't say when the offer would end. Customers can keep the unlimited data as long as they want if they sign up before the offer goes away.

The carrier stopped selling unlimited data to new customers years ago, allowing customers to keep the deal if they already had it. AT&T recently raised the price on that plan for the first time in seven years.

AT&T faces controversy and legal trouble because it used to throttle grandfathered unlimited plans for the rest of each monthly billing cycle once customers used 5GB. AT&T eventually relaxed its throttling policy so that unlimited data users would be slowed down only after 22GB in a month and even then only when the network is congested. But the carrier is fighting a lawsuit filed by the Federal Trade Commission and a $100 million fine levied by the Federal Communications Commission.

The same 22GB throttle point will apply to the new unlimited data plan, again only when customers connect to congested cell towers.

T-Mobile and Sprint still sell unlimited data plans to new customers, while Verizon Wireless does not.