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