Verizon and Sprint do not use SIM cards. The SIM card slot can be provisioned to accept a SIM card provided by the carrier (Verizon or Sprint, as appropriate) that allows international roaming via their GSM roaming partners.
The iPhone 4S is shown in the left and the iPhone 4 is shown in the middle. I am wondering if the sim card slot is empty or if it is actually physically used by either Verizon or Sprint. Just curious to find out.
It WILL be used if you subscribe to an International roaming plan from either Verizon or Sprint. It's unknown whether they are including SIMs in the shipping phones or whether you will have to get one from them if you intend to roam outside of the US.