1) They always had the option to tell FedEx to deliver on any day they wanted so that isn't a real explanation.
2) iTunes activation for home deliveries isn't going to be hard on the servers b/c ATT already has our billing info and no need for another credit check. It's the new activations in-store that are going to tax ATT servers.
3) This is really the same explanation you give in #1, worded differently. Again, Apple could have done this at anytime. It doesn't make sense they would change their marketing course overnight for that reason.
The most logical reason IMHO is that FedEx told Apple that it needed to start delivering phones as soon as it rec'd them or they might not be able to deliver them all on the 24th. I doubt FedEx is in a "Xmas mode" right now w/ staffing and a lot of drivers are probably on vaca, which means temps that don't know their route as well as the main driver.