Sprint, the third-largest cell phone carrier, has been struggling financially for several years, and its new iPhone 4S is expected to be a boon to its bottom line. And to enhance the appeal of its iPhone offering, Sprint has continued its policy of unlimited data — watch all the movies you want on your iPhone. That's good for attracting customers, but not so good at conserving network bandwidth. Now there are reports that some Sprint iPhone 4S users are experiencing pokey data speeds.