For the last 2 years I have been carrying a cell phone, an iPad Touch, an iPad Shuffle, and (when I thought I would need it) a camera. I was waiting for a device that would do it all in one, but without forcing me into an unwanted 2-year cell phone contract (I use an AT&T prepaid GoPhone account). The iPhone 4 is that device. For the most part, I adore my new iPhone. Only the substandard built-in software keeps it from being a 5-star product, IMHO. More on that later.
When I was reading reviews before my purchase I wanted to know if "factory unlocked" really meant what it said. Yes, I would have to say that it does. This was clearly a brand-new phone in factory packaging, not a phone that had been unlocked illegally. On the down side, because it had been packaged for the UK market, it came with a British-style plug on the wall charger! This didn't matter to me because I always charge my device from my computer, but the vendor should have mentioned it.
The important part was whether it was really mine to do with as I pleased. I took the AT&T SIM card out of my old Motorola phone, cut it down to size with a SIM cutter, and popped it into the iPhone. When connected to a computer running iTunes, the phone immediately started working with my same old phone number. It was that simple.