Apple has released iOS 5.1 Beta to developers and some digging in the source code reveals some new device codenames, including a new iPad 2,4 model, a new iPhone 5,1, and a device codenamed J33 which is believed to be a new next generation Apple TV, but it is unsure whether we are talking about an upgrade of the current Apple TV or the rumored Apple iTV.
It is not unusual that these product numbers / code names appear in coming iOS releases. As for exactly what they stand for is more a speculation. The reference to the iPad 2,4 model could be the much rumored iPad 3, which is believed to finally bring retina display to the iPad, or it could be for a Verizon version of the current iPad, which if we are looking more short term is more likely.
The iPhone 5,1 is most likely a reference to the next generation iPhone, which could be named iPhone 5, although some speculate that it could be called iPhone 6 instead, as it would be the 6th generation iPhone if we count number of releases from the first iPhone. More interesting however is the possible features and changes that it may include, as we have recently seen rumors of suppliers providing Apple with 4-inch screens for the next iPhone.