Verizon's 4G LTE network IS OPEN ACCESS, just some really deep technical things that I can't get into or .01% of the population would understand...
Here's the problem simplified:
VZW's strategy from the BEGINNING was to migrate their CDMA2000 network to a CDMA2000+LTE+GSM hybrid network. That process is now done in the top 100 US markets and everywhere else where there is VZW 4G LTE coverage.
That was stage one. Stage two is using VZW's native LTE network which is a full GSM 3GPP R8 core network with the latest/newest technology in the world with international roaming using DIRECT ROAMING onto the VZW 4G LTE GSM SIM & CDMA2k network with CSIM (LTE, UMTS, GSM, CDMA2000) without going through a faux Vodafone SIM card.
This will be happening with a software update in the next few months and yes Droid 4 + LG Spectrum will have it with the links or any other future LTE phone with the GSM radios on VZW. This is the back-end network upgrade VZW is talking about. Interfacing their domestic US GSM/4G LTE network with their international GSM roaming partners.
Now if you want to really get technical:
EVERY single VZW LTE phone in the US uses the GSM method of authentication of a UICC (USIM-UMTS/UICC-LTE SIM (IMS) with the ICCID onto VZW's native US LTE/GSM network) and a CSIM onto their legacy 1X/EVDO network. There is a CSIM profile for the CDMA2000 network and a USIM/IMS profile for the VZW GSM 4G LTE network. Thereby VZW implemented GSM aspects of CDMA and yes you can swap any SIM card in-between any 4G device on VZW's network.
A LTE SIM is the same as a GSM SIM in VERY SIMPLE terms just upgraded applications onto it. (CSIM=CDMA SIM card profile onto their 1X/EVDO network, USIM=VZW LTE network card profile and international LTE&UMTS roaming soon, legacy GSM EDGE roaming).
Again the VZW is delivering a GSM network, there were some deep technical things that had to be ironed out (VZW implementing/converting their entire US network to LTE/GSM, etc. GSM VoLTE MAP core voice testing late this year, next year, etc, etc. Google for IEEE and other engineering stuff if you want.)
Check
www.hoardforums.com if you want more info and I have some technical stuff that I read/work with that I can't disclose.
This is the VZW 4G LTE SIM card (also available as microSIM's in VZW stores):
http://en.wikipedia.org/wiki/UICC
One VZW 4G SIM card, all the networks in the world!
The really short answer: VZW is implementing the NEWEST GSM technology, NEWER than most countries, only with other LTE deployments (mostly developed countries) in order to gain the latest spectral efficiency due to explosive US mobile broadband demand.