iOS 5.1 Beta seems to be a treasure trove of product ids with the exposure of the iPad 2,4and the J33 codename for the next generation Apple TV. The latest part number to come from iOS 5.1 is the iPhone 5,1 as noted by both @FilippoBiga and 9to5Mac.
The progression of iPhone product numbers are as follows:
iPhone5,1 clearly represents the next generation iPhone that Apple is working on. Rumors have suggested the next iPhone will carry a 4″ screen rather than the usual 3.5″ screen found in all previous iPhone models. The new iPhone is also believed to have been prototyped with a tapering design in the past, though the most recent rumors suggest the actual release iPhone will not resemble the mockups.