Is the iPhone 4S "world phone" locked or unlocked? And why can't I use an unlocked iPhone 4S with Verizon/Sprint?

After hearing that the iPhone 4S is a "world phone" that can "roam GSM networks in 200 countries around the world," I got to thinking how does this work and what are my options for purchasing with the intent to use the phone with various networks.

When they say "roam GSM networks", does this mean that the iPhone 4S is locked to one carrier (GSM or CDMA) but can $roam$ on someone elses network while abroad at a premium price (essentially what AT&T customers could do before I think)?

Or could it mean that the iPhone 4S is unlocked and I could swap in a GSM SIM chip from another country and use their network at THEIR rates? I'm guessing the former, but a true "world phone" would be the latter.

Additionally, the iPhone 4S is included on their "About the Unlocked iPhone" and states: "The unlocked iPhone 4 or iPhone 4S will not work with CDMA-based carriers such as Verizon Wireless or Sprint."

Assuming that this statement holds true Friday and Apple offers an unlocked iPhone 4S, why doesn't it work with CDMA carriers if it has the CDMA chip to do it? I mean it's obvious that the iPhone 4 cannot, but an unlocked iPhone 4S seems that it should be able to work with either network.

I did ask an Apple Support staff through the Apple Store online and the staff person didn't seem to know anything about the iPhone 4S, particularly in terms of unlocked versions.

Thanks for your input.