Image courtesy of Apple
When Gizmodo acquired an iPhone 4 prototype, I recall a mention that Apple execs were especially annoyed because there had been two versions of the iPhone 4. The one Gizmodo recovered was the more aggressive of the two designs, with the other version prepared as a backup. When Gizmodo showed the prototype to the world, it became impossible to go with the alternate design.
When developing a backup release for any product, software or hardware, you want to minimize the resources spent on it. You spend most of the effort on the primary design, the more aggressive version. The primary will ship, its just a question of when, so the resources spent on it will not go to waste. The backup, in contrast, will ideally never see the light of day. You'd prefer to only spend resources on it which can also benefit the primary effort, and minimize the amount of throw-away work.
So what might the "Alternate Plan B" version of the iPhone 4 have been? Using the A4 CPU seems assured, as not using it would have led to uncomfortable questions. It presumably would also have incorporated the current versions of the baseband radio circuitry, and possibly the new gyroscope as well. If the gyro didn't work out, it could be marked no-stuff in assembly and the software support disabled. I'd speculate that the Alternate Plan B for iPhone 4 was a new board with A4 CPU and gyro sized to fit in a minimally modified 3GS housing.
I wonder which version would Apple have gone with, had they a choice?