I got a free replacement for my damaged iPhone 5 at my local Apple Store. I watched the clerk take a SIM card out of the old iPhone and pop it into the new one, which surprised me because I thought Sprint iPhones didn't use SIM cards.
Can my replacement phone be used with another carrier? What about a carrier that, like Sprint, uses CDMA, for example Verizon?
Even CDMA iPhones have a SIM slot. They aren't required for use on carriers that support the network protocol, but rather are used for roaming on other networks (GSM). Apple truly built a "worldphone."
To answer your questions, no, your phone is locked to the carrier it was purchased through. That applies to both the SIM (GSM) and the CMDA networks. Even though carriers like Sprint that don't operate on GSM still lock it down. And I assume they do sell SIM cards for roaming purposes.
Additionally, the replacement phone Apple gives you is indeed unlocked, which a twist. It locks itself to the first SIM card it detects. If you replaced your Sprint SIM card with one from AT&T and brought in your phone, the replacement they give you would then work (and only work) on that network as soon as the SIM was inserted as iOS had a chance to activate itself.
This makes sense as Apple wouldn't keep replacement stock for different carriers (that would be inefficient).
There's a monstrously long thread on MacRumors regarding this topic and ultimately, the conclusion was that the phones where indeed locked, just like their GSM brethren: http://forums.macrumors.com/showthread.php?t=1246693
I can't comment on the unlocked part of your question but I can answer your Sprint part of things...
Apparently, the iPhone does have a SIM when used on Sprint's network even though it's a CDMA carrier. The SIM portion is used only for travelling internationally.
There are different part numbers for the different carriers, size, and color (i.e., 3 carriers, 2 colors, 3 capacities = 18 different replacement parts from which to choose).
So, the phone that the technician had in his hand was already destined for Sprint. It probably wasn't ever unlocked. Sorry.
The SIM card is there so you can still use your phone when outside of the United States. Your phone will roam to an international GSM operator, as basically nowhere else in the world uses CDMA.
CDMA phones are generally intended only for use by the carrier they were sold by. It's conceivable that you could get Verizon to let a Sprint phone on to their network, but their policies don't allow for devices sold through other providers to be used on their networks.
