T-Mobile's version of the Samsung Galaxy S4 was originally supposed to be available for purchase on April 24th, and Sprint's version wasn't going to be too far behind on April 27th. Unfortunately, both carriers had to delay their retail launch until Monday, April 29th. There are some differences between the two. With T-Mobile, the online versions of the device will have to wait until the 29th, while the physical in-store retail versions will be available on May 1st. With Sprint, you will still be able to order their version over the phone or online on April 27th. Sprint's physical stores will have to wait until Monday the 29th.

The problem was something beyond their control and turned out to be supply issues. Here is Sprint's statement on the matter,

"Sprint is excited to launch the new Samsung Galaxy 4. We had planned to launch this next generation of the award-winning Samsung Galaxy line-up on Saturday, April 27. Unfortunately, due to unexpected inventory challenges from Samsung, we will be slightly delayed with our full product launch. Sprint is one of Samsung's largest partners and we are working closely with them to launch in all Sprint channels as soon as possible. We expect to make GS4 available at Sprint and Telesales (1-800-SPRINT1) as planned on Saturday with Sprint retail stores and other channels receiving devices as inventory becomes available."-Sprint