WHAT WILL UNLOCKING AN IPHONE DO FOR ME?

Getting an unlocked iPhone will allow you to use it on T-Mobile, where you can get a $5.99 unlimited data plan, as opposed to the $20 unlimited data on AT&T. An unlocked phone only means that you can use a SIM card from any wireless carrier, and it will work.

Here is what you get when you have an unlocked iPhone
1. The ability to use a sim card from any wireless carrier
2. Absolutely no hardware warranty or support from Apple
3. Absolutely no updating of the software without locking it again (meaning no bug fixes)
4. The possibility of the unlocking software to cause your iPhone to completely shut down and not work again (even later down the road months after unlocking it)
If all of those are things you are willing to deal with then getting the unlocked iPhone is a good deal for you.

If you buy it unlocked then you can get it put on any phone company that uses SIM cards. If you are planning on using AT&T then just buy it regular. If you want to use T-Mobile, then buy it unlocked.

you can jailbreak the phone and then use a sim card attachment to unlock it: http://www.gsmunlocklabs.com/unlock-ipho…

Well yeah, unlocking means you can transfer it to a different phone company and not just att

It will invalidate your warranty

yes probably.