To add to what Chundles said, in addition to the three iPhone carriers here (Optus, Telstra and Vodafone) having to let you unlock after six months, Optus also gives you the option to buy the phone outright and pay an unlocking fee the moment you buy the phone. For example, where I live, Telstra is the only carrier. However, I prefer pre-paid phones over being on a contract, but Telstra only offers the iPhone on a contract with no option to buy it outright. So I can go to Optus, buy the 8GB iPhone for just over $800AU, pay about $100 to get it unlocked legitly, slide in my current SIM, connect it to iTunes and have a legally unlocked iPhone using a Telstra prepaid SIM that I own 100%.
All that's stopping me is I'm not stupid enough to pay over $900 for a phone and to get it unlocked when I can get an iPod touch, which is pretty much the iPhone without a camera or phone, for $550 - which also has 4 times as much memory as the aforementioned iPhone model.