Way back in November of last year,
the Library of Congress made is legal to unlock your mobile phone. So, theoretically when the iPhone comes out, you could unlock your iPhone and use it on another carrier.
Of course, that is just in theory. More than likely Apple will include some form of protection against this sort of action.
However, there are more than enough people in the world that love nothing more than to circumvent security measures. It’s like trying to stop people from hacking Windows; no matter how long Microsoft works at it, there’s always someone out there running a hacked copy of Vista.
The reason that I don’t see this happening is that in order to get an iPhone, Cingular is going to want you to sign up for a 2-year plan. It’s going to cost a lot of money to buy one, then turn around a break your contract and go somewhere else. Then again, there will likely be people purchasing iPhones with the plan only to hack them, then turn around and sell them for outrageous prices on ebay.