For the first time in more than five years, AT&T is offering customers the ability to sign up for unlimited data.

The plans, being announced Monday, apply to new and existing customers, but require people to commit to AT&T for both cellphone and television service. AT&T isn’t saying how long the offer will be available, but those who sign up for it will get to keep it even after AT&T stops offering it for new customers.

The big question now is whether this will be a one-off deal or mark a shift back to offering unlimited data for AT&T and Verizon, which long ago stopped selling such plans. Unlimited data could be a big selling point for customers tired of data caps and looking for unfettered access to high-definition video.

However, offering such plans is risky for AT&T, as they can eat into profits: Customers are free to use gigabytes and gigabytes worth of data and have little incentive to switch over to Wi-Fi networks.

T-Mobile and Sprint still offer unlimited data options, though T-Mobile emphasizes tiered-rate data plans and Sprint has been raising rates on its unlimited data plans. Verizon stopped offering unlimited data to new customers in 2011, and existing ones can only keep their plans by buying their new phones at full price.

AT&T’s move comes as T-Mobile is offering unlimited video streaming to customers through its BingeOn service, though in its case, it is limiting the speeds and video quality. AT&T says it is offering video and other data at full speed, with the only limits coming in for those who use more than 22GB in a month. Those customers may see their speeds slowed if they are in an area of network congestion.

But AT&T stresses it didn’t make the move in response to T-Mobile, but rather as part of a broader effort to use its DirecTV deal to offer combinations that its rivals can’t. The company promises it will offer other deals this year in an effort to get customers of either TV or wireless to pick AT&T for both.

AT&T is said to be exploring ways it can secure unique mobile rights to some content and offer those to its wireless subscribers. AT&T’s offer applies to both its DirecTV satellite-based service as well as the U-verse television option that AT&T offered before it acquired DirecTV last year.

The unlimited data plans will cost $100 for the first line and $40 for additional lines. Those who buy three lines can get a fourth one for free. Television service costs extra, though AT&T says those who have wireless service will still qualify for a $10 discount on normal rates. AT&T TV service starts at $19.99 per month for the first year, though the entry-level price jumps to $49.99 after that.

CEO Randall Stephenson hinted at this last month in an appearance at a UBS financial conference. “Now what you will be begin to see — and we will begin to put some details on this, announce it and begin launching in January — [is] the idea that you put that portfolio of mobile stacked content together with a really robust wireless asset,” he said. He also played up the notion that the company would have enough spectrum to try some new things. “And keep in mind,” Stephenson said, “what we’ve done over the last three or four years is … accumulated a portfolio of spectrum for our wireless network that is unique.”