Don't get too excited
I've worked for Verizon Wireless, and while their "world" phones are, or can be, SIM lock-free, it only works outside of the US. You can't run what Verizon calls a worldphone (CDMA + GSM combo device, like certain Blackberrys and HTC devices) on a US carrier, even if it's got its SIM lock removed.
Preventing certain carrier IDs from being used regardless of SIM locks is super trivial to implement.
Conversely, you can't run an unlocked Euro iPhone 4s on Sprint or VZW either, because those are lacking the CDMA portion. You could run it on at&t, or, at GPRS/EDGE speeds, T-Mobile US.
And lastly, swapping a CDMA-enabled iPhone 4s between VZW and Sprint won't work either, because it's only the GSM portion that will be unlocked, whereas CDMA carriers tie the phone to the subscriber via ESN and MEID (electronic serial number and mobile equipment ID), and they do keep track of these, i.e. only equipment sold through the respective carrier will be allowed to connect to that carrier's network.
All this means is "you can put a foreign SIM into your VZW/Sprint iPhone 4s, removing the disadvantage US CDMA iPhones would have had compared to at&t ones." Don't read anything else into this.