I've run into a few Sprint / Verizon iPhone 4 and 4S that have been unactivated, as well as quite a few people who were having trouble activating these phones due to them not having any service setup for the phone.
This may be somewhat well known by now, but I see this 4S forum seems quite empty and through ah hell I'll contribute.
Simply plug your iPhone (4 or 4S on iOS 5) into your computer and open iTunes, it will begin the activation. When it asks for your Sprint / Verizon information, enter any kind of bullshit info at all, and click continue. The activation will fail, but your phone will have been taken out of activation mode and sent to the Home Screen.
On the iPhone it will say (Activation Failed), just tap "ignore" and enjoy messing with your expensive iPod