The file, /System/Library/AppleUSBDevice/USBDeviceConfiguration.plist, lists USB configuration details about various iPhone and iPod touch models. Currently released devices listed are the iPhone1,1; iPhone1,2; iPod1,1; and iPod2,1.
Steven has found references to four unreleased devices the iPhone 2,1; iPhone3,1; iPod3,1; iFPGA; and iProd0,1. All of the products are assigned unique productID numbers but have a productString of iPhone.
ArsTechnica suggests that the iPhone3,1 and iPod3,1 are clearly references to next-gen hardware version of those products (perhaps Apple is skipping iPhone2,1?). Smith suggests that the iFPGA could be a prototype device that uses field-programmable gate arrays, a type of programmable microchip. It doesn't conform to the standard numbering scheme that Apple uses for its hardware products, so it isn't likely something intended to be released. The "iProd," on the other hand, uses a 0,1 number, suggesting it is a prototype of an as-yet unreleased device that is neither an iPhone nor an iPod touch.
Could iProd be the rumored 10" touchscreen?