January 29, 2009 MacRumors has found a reference to "iPhone2,1" in the iPhone's 2.x Firmware. What makes this special? The original iPhone is "iPhone1,1" while the iPhone 3G is "iPhone1,2" - which points to a major hardware revision in the works.
As to what this major hardware revision might entail, your guess is as good as ours, but we can tell you that the iPod Touch gained a speaker, volume controls, a microphone and a bump from 412MHz to 532MHz during the jump from "iPod1,1" to "iPod2,1".
With the first iPhone released on June 29, 2007, and the iPhone 3G on July 11, 2008, we're fairly sure you'll know a lot more about this unannounced iPhone by mid 2009.
Got any bright ideas about what hardware capabilities Apple is adding to the iPhone, or just what they should add? Let us know in the comments!