Mac Rumors has discovered that the latest iPhone OS update contains evidence that points towards a new iPhone.
The code strings which can be found in iPhone OS version 2.2.1 designate an iPhone with the model number "iPhone2,1". For reference the current iPhone 3G is model "iPhone1,2" while the original iPhone is "iPhone1,1".
The complete change in model number would seem to suggest a possible significant hardware upgrade in the future.
This new model number can be found in the USBDeviceConfiguration.plist in the update.