If you're not into the whole jailbreak thing (assuming there will eventually be a iOS 5 untethered jailbreak) and you want an unlocked iPhone 4S, fear not. Apple will be releasing an unlocked version of the iPhone come November.
Bad news for CDMA customers though -- you can't use the unlocked iPhone 4S on CDMA networks. The unlocked iPhone is only good on GSM networks such as AT&T and T-Mobile (in the US).
As far as GSM goes, however, you can use the unlocked iPhone 4S on any GSM carrier simply by inserting a micro-SIM provided by that carrier.