That is just silly /ubbthreads/images/graemlins/tongue.gif
Ford could not and would not do this even if they wanted to! It is your truck, not theirs, they have no legal right to block your ability to add power upgrades to your truck. What's more, I don't see why FOrd would care if you did, because all it does is give them the ability for a legallly and morally correct "out" of fixing your truck on warranty. Saves them lots of money, voiding warranties for tuners and chips.
Furthermore, there really isn't any way to make a PCM "immune" to a module, since all it does is modify the signal sent from a sensor to the PCM. The PCM doesn't and couldn't know the difference.
The only way that they could keep a tuner from working is to add a "passcode" in a sense to the "ford approved" prgramming, that would not allow any "non approved" programming to be downloaded. Again, i cannot imagine that Ford's legal department would allow them to do this, because, again, it is your truck, not Fords, and they have no legal right to deny you access to any part of your truck that you want to access. They can and will void your warranty for accessing parts that they don't want you in, but they cannot keep you out. It is your truck, your choice.