Unlocked iPhones can run apps which Apple bans from its App Store. This can give your iPhone additional functionality. In addition, unlocked iPhones can be used with other phone companies. This means that, for example, in the United States, you could take your iPhone to a cellular service company other than AT&T.
However, you do risk ruining your iPhone. Apple likes to release devious little 'patches' that detect if your iPhone is unlocked, and these patches can destroy your phone and render it unusable. Many (though not all) of the features that people jailbreak their phones to get will eventually be released by Apple. It's not recommended, unless you're very tech savvy, or have money to spare.
You can visit apple.com or visit a local apple store for unlocked iPhone.
Unless its an iPhone then it would be no.
You can check to see if your iPhone is unlocked by inserting a SIM from a different service provider and try to make a call, if its successful. Your iPhone is unlocked.
eBay or Craigslist
Yes, you can now buy the iPhone 4S unlocked direct from the Apple website
yes. once an iphone is universally unlocked it will work.
There are several providers of unlocking services for iPhones - both on location in Orlando, but if you don't have an unlocked iPhone yet, you could check out unlocked iphones on www.unlockediphones.net.
600
Yes
No!
No it has to be unlocked
Yes, you need an unlocked & Jailbroken iPhone. Which isn't cheap.