In the US, major carriers Verizon and Sprint have operated CDMA-based networks whose devices did not use SIM cards. I think it was the iPhone 4 that Apple once shipped 3 different versions of in the US: AT&T (GSM), Verizon (one set of CDMA bands), and Sprint (another set of CDMA bands).
But did you notice that Sprint is supported in this announcement? It is because LTE does in fact use a SIM, even on the carriers that rely on CDMA for voice. Fortunately for the iPad, it has no voice requirement.
In reality, there is the technical means to support this on ANY carrier. You just set the IMSI or ESN in software on the microprocessor and then hit an API at the carrier telling them to pair X device with Y unique ID. The difference now is that no one before Apple has had the weight to get the carriers to go along with this, since it encourages modularity and discourages device and carrier lock in.
