Well older VZW phones with SIM slots had them specifically for international roaming, as stated, and you didn't need anything there. As LTE is a GSM standard, however, Verizon has elected to use SIM cards for all LTE devices, and used them for CDMA identity as well (likely following Japan's model of using uSIM's for their CDMA devices). Considering that, I see no reason to expect Verizon wouldn't do the same thing here, especially considering it will eventually support GSM roaming. As far as the SGS3 using a different SIM, I don't buy it. There are 2 SIM sizes used now, mini-SIM, which is being phased out, and micro-SIM, which all the newer LTE phones and most GSM phones are now using. For those of us who already have LTE microSIM's, I see no reason we should expect them not to work.
Sprint's LTE phones don't use SIM cards for the same reason Sprint is selling LTE phones without having announced when the LTE network will go live.