Factory Unlocked iPhone's for tMobile and MetroPCS?

Hi, me and my sister want iPhone's for christmas but we have TMobile and Metro PCS, but we could get used Factory unlocked ones off of craigslist, right? Factory unlocked means that we can use TMobile or MetroPCS on an iphone, and also, would we still have all the features? Like be able to update our phone, and listen to music, and go on the internet? And if we update our phones with iTunes it won't take off the "factory unlock" correct?

If you get an AT&T iPhone 4/ 4s you don't need to unlock it to switch it over to to Straight Talk. You do have to go through a special process to change the APN settings from AT&T to ST but don't need to "unlock" it. You can do this with the other carriers too but you need to know their APN settings to make the change.

You will have all the same abilities to update the phone as the phone is not that of AT&T it is that of Apple so once you adjust the APN settings you have adjusted the phone and this is okay to do. There's a chance that AT&T could discover the change and reset them to their APN settings however not likely.

If the phone is off contract and you are or ever were an AT&T cost AT&T will unlock it for free but you have to be the owner of the iPhone or talk to the person selling it.