Essentially, Apple wouldn't have to worry about bundling carrier-specific SIMs with the iPhone and end-users would have only virtual contact with the carriers. Something Google tried to do with the Nexus One.
This would make the overall purchase of an iPhone from Apple's Online Store, or Apple's brick-and-mortar stores, a much simpler and smoother process for the end-user -- whilst cutting out the retail aspect that carriers currently hold. Although, I can't imagine carriers not wanting to sell the iPhone directly from their retail stores since the iPhone is such a hot commodity these days.
It'll be interesting to see how this one pans out.