[ez-1]

I think there's two different ways to unlock it and just bhara said there is no free way to unlock 3.1.3. But you haven't got that phone yet have you? If you don't have it yet then just make sure that you don't update it straight away. When you connect it to itunes it will ask you to update but just ignore it and go with unlocking. I'll post you some links if you haven't found them yet.

So go to iclarified.com and brows your self to tutorials. There you have plenty of good stuff... I would go with blacksn0w.