Wireless carrier T-Mobile and discount retailer Walmart will soon offer consumers a tempting service plan: Unlimited wireless data for only $30 per month—and without any long-term contracts.

The joint announcement. made yesterday, adds upon the low-cost smartphone plans previously announced by T-Mobile and Walmart. But unlike those plans, the new plan seems to emphasize wireless data over talk time.

The plan offers only 100 minutes of talk with overages charges of 10 cents per minute. But text messages are unlimited, and you'll get 5GB of wireless data delivered at T-Mobile "4G" (really HPSA+) speeds. (As with other T-Mobile service plans, data speeds are reduced once subscribers go beyond their allotted monthly amount.)

T-Mobile's latest contract-free plan, expected to hit Walmart stores and online at Walmart.com, is an example of the growth in prepaid plans. (See "No contract options multiply".) And it certainly seems to reinforce T-Mobile's position as a low-cost wireless service provider—one which would have disappeared if its merger with AT&T wireless wasn't opposed by the Federal Trade Commission.

But as our latest Ratings of wireless carriers (available to subscribers) show, T-Mobile's cellular service isn't the most highly regarded by the thousands of Consumer Reports subscribers in 23 cities who responded to our annual survey.

To avoid "bill shock" and save money on your wireless phone service, see our report: Best cell-phone plans and providers.

Walmart and T-Mobile Introduce Exclusive No-Annual Contract 4G Offering [T-Mobile press release]
Walmart, T-Mobile to Offer $30 'Unlimited' 4G Prepaid Plan [PCMag.com]
Wal-Mart, T-Mobile offer cheap prepaid plan [Washington Post]
T-Mobile's $30 'unlimited' 4G all-in-one plan, coming October 16 [ZDNet]