Well, there's all these rumors about a new hardware SKU showing up in app stat data provided to developers. They've been seeing a steady increase of instances of "iPhone2,1" showing up. For those unfamiliar with the product identification numbering scheme:
iPhone: iPhone1,1
iPhone 3G: iPhone1,2
So the existence of an "iPhone2,1" would indicate a hugely substantial hardware change to differentiate it from previous models, as a change in the first numeral has always indicated a substantial change to the main chipset/board on their other products, whereas the 3G was merely a change to baseband components (3G and GPS) and appearance.