[zred]

CDMA phones come with an ESN (http://www.mobileburn.com/definition.jsp?term=ESN). It's basically like your SIM card number, except that it doesn't change.

CDMA carriers can activate a phone not sold by them by ESN. However, some carriers have decided that they would prefer you to buy a handset from them and refuse to activate devices that weren't sold by them. Sprint has a policy of not activating any ESN that comes from a phone they didn't sell. I'm not sure if that applies to the iPhone since a non-Sprint iPhone would really be the same device. Anyway, unlike SIM cards, you're more at the mercy of whether a carrier will activate it or not.

[sklnd]

ESN is equivalent to IMEI in GSM/UMTS networks. IMEI is tied to equipment, and never changes, just like ESN.

CDMA networks use IMSI to identify subscribers (as opposed to the equipment serial number) just like GSM/UMTS. IMSIs are provisioned on to the phone by the carrier with special software supplied by the phone manufacturer, instead of being supplied to the phone by the phone asking the SIM for it.

There's no technical reason why CDMA networks can't use a removable card to store subscriber data, and indeed there are a few standards for doing so (RUIM and CSIM) and a few carriers that use them.