Sprint will reportedly introduce a 5GB cap on mobile hotspot data for smartphone users beginning October 2nd. A purported leaked document obtained by SprintFeed states that Sprint’s $29.99 Wi-Fi tethering add-on for smartphones will no longer be unlimited beginning early next month. Additional data above the 5GB cap will be billed at $0.05 per megabyte according to the document. The new policy would not impact standard smartphone data, nor would it affect the mobile hotspot function on tablets. This is a slippery slope, however, and it could indicate that Sprint’s unlimited data plans are eating too much the carrier’s margins to be sustainable. Sprint has told BGR on numerous occasions that it will continue to offer unlimited data plans for as long as it can, and the carrier is expected to launch Apple’s iPhone 5 next month with unlimited data. Sprint did not immediately respond to a request for comment.