Ars points to a USBDeviceConfiguration file which lives in the iPhone SDK 3.1 that has a surprising new addition. Included in all of the recent iPhone SDK updates until now has been the iProd 0,1 device file. 0,1 device files typically stand for prototypes in Apple hardware lingo.
As of iPhone SDK 3.1, there is now a new device profile for an iProd 1,1. Theoretically, this means that production "iProd" devices are already being trialled on iPhone OS 3.1. Ars speculates that this could be the tablet or perhaps even an iPod nano with a camera that would run the iPhoneOS (and would therefore have a lot of other goodies).
Frankly, its hard to decide which of these scenarios is more exciting.