The unlocking process for Apple iPhones is more complex than most other phones and it can take up to 72 hours to unlock an Apple iPhone.

To unlock an iPhone you will need to contact the phone network that you purchased your iPhone through. They will ask for your phone’s IMEI number, which you can find out by going into your phone’s settings (Settings > General > About) and within a few days you will receive a text message saying that your phone has been unlocked. Please note that this text message doesn't mean that the unlocking process is complete. You will still need to connect to the latest version of iTunes (with your new SIM card in the phone) to restore your phone. If you do not restore in iTunes, your unlocking process will not be complete.

Once your iPhone has been unlocked, you will see a screen like this in iTunes:


Most iPhones purchased from Apple Stores are sold unlocked and are ready to use with any SIM card, however it is still best to check by inserting a SIM from a different network as some Apple Stores overseas still sell locked iPhones.

Here is some information from the Telstra website about unlocking your iPhone. This link has details about unlocking your iPhone along with the contact details at Telstra for their iPhone unlocking team.

To unlock an iPhone on the Optus network you need to call 1300 075 395. This is a 24-hour service for processing iPhone unlocking requests. You will need your phone's IMEI number, the Optus phone number that you normally use on your iPhone as well as your date of birth.

Here is some information from Virgin Mobile about unlocking your iPhone.

These two things are really important. You won’t be able to use the T-Mobile USA SIM card (or any other US SIM card) if your phone is locked and not compatible with the US network. It should only take 5-10 minutes or so to check these things, and doing this will avoid any dissapointment when you arrive in the United States.