As might be expected, it doesn't seem that the iPhone 3G will be Apple's last iPhone.
MacRumors has discovered code inside the OS X 2.2.1 firmware released earlier this week that describes an "IPhone 2,1" model. Previous discoveries have shown that Apple uses this labeling nomenclature for both the iPhone and the iPod Touch.
The original iPhone was labeled "1,1", while the iPhone 3G was model "1,2." The iPod Touch has already advanced to a second-generation design, as the iPod Touch released at September's music event received the "2,1" label inside its firmware.
Little else is known about iPhone "2,1." It's reasonable to assume that since the iPod Touch number changed significantly with the addition of a new processor, that such an upgrade might be coming to the next iPhone. P.A. Semi has been working on iPhone chips since last spring, and other evidence has surfaced indicating that Apple might be planning to upgrade the iPhone's graphics sometime in the near future.
Apple marketing chief Phil Schiller seemed to imply earlier this year that Apple was locked into a June/July upgrade cycle with the iPhone, noting that Macworld's January date conflicted with a number of Apple's regular product cycles. So, perhaps another summer iPhone release is on tap.
Before that, however, I still think there's a solid chance Apple doubles the capacity of the iPhone and iPod Touch without making any other major changes, just as it did last February.