According to news sources, Apple is working with security firm Gemalto to develop custom, writeable SIM card for future iPhones. If this goes well, this means that the SIM can be reconfigured to work on different carrier by simply uploading a small file with carrier-specific data to the module’s flash memory. A phone call is all that is needed to update the internal SIM to work with different carrier’s network.
Apple will also no longer need to worry about the device being exclusive to specific carriers; no more SIM lock. While this could initially be offered in the European market, it is likely to be made available in other countries in time to come.
Source: Ars Technica