It's definitely possible. There is a specification for a programmable GSM SIM that Apple was involved in creating. There is also a dual radio chip set that does GSM and CDMA, so no SIM and dual network are both possibilities.
Note that not all carriers are ready to handle an embedded SIM card, so there may be some acceptance issues depending on the programming mechanism. If Apple does something really revolutionary and allows carriers to program the SIM through an Apple website or something like that, it would speed adoption.
-t