And the SIM only gives you access to the LTE data network.. not voice calling capability.
Verizon does not do cellular voice over LTE.. all that still goes over the 1xRTT network, which requires an authorized ESN in the traditional manner..
Actually, the voice goes over EVDO (3G). That's how the Tbolt can do simultaneous voice & data (or SVDO). In that case voice uses EVDO and data uses 1xRTT. Verizon could be doing this with all of their phones, but the reason they haven't (and the reason they haven't openly advertised it) is that they feel it would not be a great user experience. While I can understand that (1xRTT is very slow) they should at least give the option.
As far as the LTE SIM, I realize they are not doing voice over LTE (yet) but after you call the 800 number to set up your first LTE device (which registers the SIM on your account), you only need to transfer the SIM card to your next LTE device to switch devices. You do not have to call in every time to a CSR. I assume the SIM has access to the devices ESN when plugged in. Then, since the SIM is registered on your account, it activates the phone on the network that way.
If you go back to a 3G device, you'll have to call in to a CSR to get it accomplished (which will also render your LTE SIM void). The if you decide to go back to an LTE device, you'll again have to call the 800 number to register your new LTE SIM. But again, once you call in to register your LTE SIM the first time, you only need to transfer the SIM from one LTE device to another to swap devices.
I know this because I had the Tbolt, then to a Inc2, & am now looking at a Vigor. That is how the CSR explained what would happen.
On a side note, while VoLTE (voice over LTE) has not been implemented yet, the LG Revolution is capable (it was demoed at MWC2011). During the demo, they said that the absolute earliest we could see this on the customer side would be mid to late 2012. At the rate Verizon is pushing their LTE network, I wouldn't doubt that possibly ending up being a reality.