AT&T has ended its unlimited data plans, but U.S. mobile phone users on Sprint now must be asking if their carrier will hold out much longer. Sprint offers unlimited data plans to new and existing customers, and AT&T customers who use more than three gigabytes of data may now be asking if it's worth it to switch. That's because AT&T has put a speed cap on heavy data users, claiming its network is too crowded. There are 17 million AT&T customers who have been grandfathered into the now defunct unlimited plans, but if they go over the cap now, their phones will become virtually useless on the Internet.

Phone calls and texts still go through, but AT&T will throttle download speeds once users hit the cap, so the unlimited data is rendered meaningless. However, there is a reason people aren't jumping over to Sprint so quickly. They have no 4G network. Download and upload speeds are slower on Sprint, but the company does have a plan to build an LTE network. Sprint is advertising a plan to go live with the new network this summer in six U.S. cities, mostly in the south.

Sprint is even openly courting AT&T customers on its Web site, specifically mentioning AT&T rate increases and the fact Sprint still has unlimited plans. Once Sprint's LTE network goes live, could they put an end to unlimited plans too? By the time summer hits, anticipation for the iPhone 5 could be at hand, and that will mean more customers on the Sprint network. In fact, the iPhone's popularity may be the number one reason AT&T has so many unlimited plan customers in the first place. For many years, it was the only carrier to offer the iPhone at a subsidized price.

Added to all of this is the fact instant messaging is starting to take its toll on SMS messages that are so lucrative to the carriers. More and more companies are moving to provide easy ways to send free IM's, and it's one more problem Sprint will no doubt face as they remain the only unlimited data carrier. Let us know in the comments if you think Sprint will keep the unlimited plan for more than another year or so.