I plan on getting an iPhone 4 w/o contract tomorrow. I called the store and they are willing to sell to AT&T customers. The thing is; my current two year agreement with AT&T expires in 3 days, and I plan on terminating my plan and switch to prepaid sims. So my question is: do prepaid sims work with the iPhone 4? And, does it have to be an unlocked piece, and can I unlock it myself?