The unlocked iPhone 4 or iPhone 4S will not work with CDMA-based carriers such as Verizon Wireless or Sprint.
Sounds like that's a no-go for the 4S. Which sucks because my wife wants to replace her Android phone, but we're not upgrade eligible from Verizon yet. So that means I'm going to have to pay same price as the unlocked phone, but it's going to have the GSM side locked out if I ever want to sell it.
Footnote 2: CDMA available only if iPhone 4S is sold and activated for use on a CDMA network.