AT&T is the latest US mobile carrier to announce a new "unlimited" data offer. Starting tomorrow, AT&T will sell unlimited data for $100 a month "to all consumer and business postpaid AT&T wireless customers," the company said in a press release. Previously, AT&T only offered unlimited data to mobile customers who also subscribed to DirecTV or U-verse TV.

The single-line price is higher than AT&T's competitors are charging, and the plan has some limits despite its name. Unlimited data can only be used on the mobile device itself because the plan includes no tethering, an AT&T spokesperson confirmed to Ars. Customers who want to use their phones as hotspots will thus have to buy one of AT&T's capped plans.

By default, the new unlimited plan also has video quality limits. The announcement says that the "plan includes Stream Saver," a reference to a system that reduces video quality to about 480p resolution. Stream Saver was originally intended to help customers use less data on plans with data caps, but in this case it's being applied to customers who don't have strict data caps.

Unlimited data customers will be able to turn it off, however. "You are not required to use Stream Saver. You can turn it off once and it stays off," the AT&T spokesperson told Ars.

By leaving throttling on by default, AT&T will be able to minimize bandwidth usage on its network while giving customers the option of streaming in high definition.

The AT&T unlimited plan will cost $100 for the first line plus $40 for each additional line, AT&T said. Buying a fourth line will provide a $40 credit, reducing the monthly price to $180. The credit doesn't begin right away, so four-line plans cost $220 for the first month or two.

AT&T customers might get throttled during periods of network congestion when they use more than 22GB a month. That's pretty standard in US mobile networks, but the plans are still described as "unlimited" because there are no data caps and overage fees.

This has been a big week for unlimited data, which for a while seemed to be going extinct. T-Mobile and Sprint were already offering unlimited plans, and then Verizon Wireless on Monday announced an $80-per-month unlimited plan that does not restrict video quality and comes with 10GB of high-speed hotspot data. T-Mobile USA responded by adding 10GB of high-speed hotspot usage to its $70 unlimited plan and by giving customers the option to stream video in high definition without paying extra. Sprint then improved its own unlimited plan, charging $50 a month and adding HD-quality streaming video and 10GB of mobile hotspot usage.