My understanding is that it means Verizon and Sprint have said no to allowing iPhone 4S' that haven't been specifically bought with Verizon or Sprint service onto their network. So you may call to add your phone to their network, and they may reply 'sure... but only if you sign up to a 24 month commitment'.
Not just in US. apple.uk and apple.ca also said this phone will not work under CDMA, don't know why. I don't believe all CDMA carrier in the world will refuse iphone 4s? uk and canada don't sell locked iphone from the first day.