Wireless carrier AT&T has long been complaining that a small portion of mobile data users have been taking advantage of its all-you-can-eat data plans, creating network congestion and driving down service levels for everyone. Now the company is doing something about it: beginning June 7, AT&T will be abandoning its $30/month unlimited data plan for a two-tiered system that gives users 250 MB or 2 GB of data per month for $15 and $25, respectively. Also, at long last, AT&T is planning to bring tethering to the iPhone in the United States, so users can connect laptops and other devices to the Internet using an iPhone’s 3G service…although that feature will wait until iPhone OS 4 is available.

“AT&T helps mobilize everything on the Internet—your favorite Web sites, TV shows, music, games, and social networks,” said AT&T Mobility and Consumer Markets president and CEO Ralph de la Vega, in a statement. “To give more people the opportunity to experience these benefits, we’re breaking free from the traditional ‘one-size-fits-all’ pricing model and making the mobile Internet more affordable to a greater number of people.”

AT&T’s new mobile data plans are dubbed “DataPlus” and “DataPro.” The DataPlus plan offers 200 megabytes of mobile data for $15 per month; if users go over that limit in a monthly cycle, they can get an additional 200 MB for $15. AT&T says this plan will cover the needs of about two thirds of its smartphone users, and represents a savings of $15 per month over their current billing.

The DataPro plan offers 2 GB of wireless data per month, and customers who go over will pay $10 for each additional gigabyte in the billing cycle. The good news here is that customers are unlikely to bankrupt themselves through indiscreet use of wireless data services; the bad news is that if you’re in the minority of AT&T customers who use more than 2 GB of data service per month, your bill is going to increase.

AT&T also plans to bring tethering to the iPhone and other smartphones as an $20 per month adjunct to the DataPro plan.

For Apple iPad customers, the 2GB/$25 per month plan will replace AT&T’s current $29.99 unlimited data plan; iPad customers will continue to operate on a pre-paid contract-free basis. Current iPad owners can stick with the $29.99 unlimited data plan if they like, or switch to the equivalent of the DataPro plan.

AT&T’s tethering option in the DataPro plan is reasonably in line with market rates from other carriers; however, heavy data users may want to check out AT&T’s business-oriented DataConnect plan, which offers a price cut on larger amounts of data transfer.

Overall, AT&T’s pricing changes should bring actually billing cuts to many smartphone users, which may encourage fence-sitters put off by the high cost of plans to try out the devices. However, folks who have been living large on mobile data may suddenly find themselves paying more—even a lot more—to pull down all that music and video.