This is really an interesting feature (let's say 'feature' at this moment). Actually, in person, I do really consider this is an tremendous improvement for any carrier-required mobile devices.
It makes mobile devices really "mobile". Customers do not have to physically enter a local carrier store to add a new line/data-plan or transfer to another carrier. It might save a great amount of time and efforts especially when traveling overseas.
Hope it came to iPhone in near future. Due to the easiness of switching carriers, hope it would help bringing down prices.
Mac App Store is really helpful especially when updating installed applications. Just a click away rather than launch every one to find out whether they need update or not.
Can't wait for the invitation.