Here is what I've discovered so far:
1: Removal of the Sprint sim card the phone retains it's connection to Sprint and functions normally. Though it does warn you that no sim is installled.
2: Removing the Sprint sim and replacing it with an AT&T sim from an iPhone 4 results in a brief flash of the "Carriers" setting before launching into activation process exactly as shown by fox777.
3: Removing the Sprint sim and replacing it with a prepaid Rogers sim from Canada that I used on my unlocked (ultrasn0w) iPhone 4 results in the phone connecting to T-Mobile on Edge (as you'd expect) and attempting to load a website via Safari redirects to the Rogers site. So, apparently it is working normally as far as I can tell.
The "Carriers" setting is available and permanent, but if you try and select AT&T the phone will not connect. Perhaps this is due to some restriction on AT&T's part, though I'm not sure.
In any event, it looks like the phone indeed is unlocked at this time, unfortunately, I am too far from Canada to test it's operation on the Rogers network. I will try to get access to a T-Mobile sim and see if it fully connects and behaves normally.
Don't know when Sprint will push this update to lock the phone, but it's kind of bummer. I suppose it is necessary to keep people from travelling from abroad to scam subsidized iPhones from Sprint, so I can see why they would want to restrict the unlocking to dedicated customers in good standing. I just hope they make good on their pledge to unlock. I would really think that since they are basically banking the future of their company on their ability to attract iPhone customers they certainly would, but, you never know. Corporate willingness to alienate their customer base seems to know no bounds.