Sorry if I'm in this thread a lot but I once again have more information to share.
Does Sprint pair the SIM to your iPhone?
Yes, and no. Sprint uses two different types of SIM cards for the iPhone and are two completely different types of card and are completely incompatible with each other.
1. USIM(Universal Subscriber Identity Module, Part No. SIMGLW406R): This is a Nano SIM card that contains ONLY LTE access information. Your phone number, CDMA services, and account information(billing, etc) are NOT stored on this SIM. 1x and 3G services as well as your phone number reside in the actual iPhone itself. However if you remove your SIM, you cannot use any cellular services AT ALL. The iPhone 5 requires the USIM to be installed to use CDMA services even though CDMA does not require a SIM card. If you try to swap SIM cards, nothing will happen. Your phone number is not stored on this card so if you change your SIM you will still have your original phone number as well as billing account. Even though the Sprint iPhone 5 is configured this way, you can still use international CDMA UIM cards if your device has been unlocked for international use. But swapping the Sprint SIM with another Sprint SIM will not have any affect on the device.
2. CSIM(CDMA Subscriber Identity Module, Part No:SIMGLW406/410C)
This card contains ALL information pertaining to both the Sprint network and your account. CDMA services are dependent on this card and cannot work without it, similar to Sprint's USIM card. You can place your Sprint SIM into any Sprint-branded CSIM compatible device and use service immediately. With this system, Sprint is NOT requiring the MEID and ICCID to be on the account to use service. You can also activate a Boost or Virgin CSIM and install it into a Sprint CSIM device and get both CDMA and LTE service.
Basically, the Sprint USIM card for the iPhone 5 doesn't do anything other than tell the phone to use the PRL in the modem for CDMA as well as tell it where to look for LTE towers. It does NOT contain ANY account information. It literally just tells the phone where to look for towers and switching SIMs will have no effect.
The only reason Sprint is using CSIM for Apple products is because Apple refuses to support Sprint's parity ************* anymore and Apple is forcing Sprint to develop a special SIM card for iPhones. Let's hope Verizon sells some of it's lower C block 700 MHz LTE to Sprint so they'll be forced by the FCC to keep 700 MHz LTE capable devices unlocked(All Sprint iPhones support this band except the 4s), and it'll help Sprint expand too. But it's unlikely because Verizon is in bed with the FCC and will do whatever they want. I doubt Masayoshi Son is a big fan of unlocking, SoftBank doesn't even offer unlocking and it's expensive as hell to submit a SoftBank IMEI to Apple's unlock database.