AFAIK, this is how the iPhone 3G / 3GS units were handled at the Apple store in Canada (at least the Bell & Telus handsets). In the box they are technically unlocked, but the store would only sell you one once you specified which carrier you wanted. At that point, they would activate it and the phone would be locked on that network. These units were never promoted as being "unlocked" so it was perfectly fine for them to do this.
With the iPhone 4, Apple is marketing it as an unlocked handset (and even mentioning the phone can be used with SIM cards from International carriers). As such, if the phone did end up getting locked to the first carrier it was used with, there would be an uproar.
Bottom line: the phone is unlocked and you would be able to use it with any carrier - and switch when needed.
With the iPhone 4, Apple is marketing it as an unlocked handset (and even mentioning the phone can be used with SIM cards from International carriers). As such, if the phone did end up getting locked to the first carrier it was used with, there would be an uproar.
Bottom line: the phone is unlocked and you would be able to use it with any carrier - and switch when needed.