According to GigaOM, Apple is developing an integrated SIM card that can work across many different networks. Presently, SIM cards are issued individually through carriers, but a phone with this new type of SIM would be carrier-agnostic when purchased, so users would be able to activate it using desktop software or a special application on the App Store. This would allow Apple to sell the device directly to customers, and completely cut out the middleman.
However, if Apple is doing an end run around the carrier by putting its own SIM inside the iPhone, it could do what Google with its Nexus One could not, which is create an easy way to sell a handset via the web without carrier involvement. Much like it helped cut operators out of the app store game, Apple could be taking them out of the device retail game.
As noted by GigaOM, this system would work extremely well in Europe, where many different carriers are able to sell the iPhone. (Though, Apple would probably implement the technology in US phones as well, to keep the hardware unified.)
This would also effectively stop users from using phones on unsupported carriers – like T-Mobile – and could be a hinderance for users who frequently swap SIM cards between devices. Since a system like this has never been tried, it’s unknown how it will affect the end-user.