The four data tiers are described in a T-Mobile brochure as "unlimited" although an added note points out: "If you use more high-speed data than your plan covers, we simply allow you to access the Web at up to 2G speeds."

Other U.S. carriers, including AT&T, which is seeking , tend to charge more when a data limit is exceeded while keeping the speed the same. In fact, T-Mobile had been adding an overage charge of 10 cents per megabyte for its 200MB data plan first introduced last November. The carrier eliminated the overage charge today on the 200 MB plan and instead implemented the speed reduction, a spokeswoman said.

A previous $30 unlimited data plan had been subject to a speed reduction at 5GB of wireless service, although that speed reduction was apparently not widely known.

In all, the four new T-Mobile plans for data are: $10 for 200MB; $20 for 2GB; $30 for 5GB and $60 for 10GB.

The spokeswoman said that when a limit is reached, T-Mobile will send customers a free text message advising them of the limit and giving them the option to change their plan to a more expensive one with more data, or continue using their current plan with reduced data speeds until the reset of the billing month.