I've never really trusted carriers in general. For devices that are exclusive to a carrier (and usually contain carrier branded OSs and junkware installed), they probably have a much better handle on supply levels, etc. For the iPhones, Apple controls the manufacturing chain and sells to carriers. As such, I decided a couple of years ago that dealing directly with Apple 1) put me closer to the manufacturing chain, and 2) was a much more pleasant experience than dealing with the carrier. Others may have a better relationship with a particular carrier or carrier store, but I've never had that. Also, since the carriers did away with any kind of "early" upgrade (like you used to be able to do after 18-20 months into a two year contract), I had even less of a reason to buy an iPhone from them. And when it comes to launch day, the only entity that will know how many units are available is probably the one making them, so Apple gets my business.