The Epic4g might or might not sell out the day it arrives, but I'll bet there WILL be a bit of a crunch come October 10th. It's one thing to say 'screw it' and burn $200 extra to get a new phone at the 6-month point. It's another matter entirely when the cheap update anniversary is less than 2 months away. The *danger* (to everyone planning to buy at the first discounted opportunity in October) is that Sprint might hold off on Epic4G orders until they see how well they sell at launch and do during their first week, and not have batch #2 ready to fly off the shelves when Mass Upgrade Day arrives.
It makes me extra thankful I was able to metaphorically cut in line and get my Hero a day early. If it really does come down to a shortage, being able to skip ahead of everyone with an October 11th upgrade date and get mine on October 10th might end up making the difference between having a very, very happy birthday that weekend, and being pissed & miserable because my new phone is indefinitely backordered.