You can unlock iPhone 3.1.3 but only if your baseband has not been updated to 05.11.07 or newer.

If you did update threw itunes, or got the phone with version 3.1.3 from the factory. Then i am sorry to say that you have to wait untill they come up with an unlock solution. It's been around two months since they sayed "The unlock will be released soon" , but nothing yet.