Well, I think I answered my own question.
I've spoke to some GSM chipset manufacturers and they indicated that it is very possible to store the SIM card contents in an EEPROM or some other non-volatile memory, thus removing the need for the SIM altogether.
Unfortunately, it appears, at least in North America, that the GSM handset approvals (specifically PTCRB) will not approve a design that does not have a SIM. The reason is purely political, and the wireless carriers simply like the idea of SIM cards and don't want to deviate from that.