yakuja as previous poster said (hard to spell the name :)) you cannot unlock iphone with OS 3.1.3 and baseband 05.12.01 at this moment. keep checking the Dev team blog (http://blog.iphone-dev.org/) as they will release probably next month as OS 4.0 iphone will be released. Meanwhile i suggest you to buy any py-as-you go (post paid) number if you can't live without your iphone.

I beleive you can downgrade 3g baseband by fuzzyband and then unlock. (i thought you said 3GS so had to edit th response)

first jailbreak iphone by spirit (download from above link) , you will have cydia in your iphone search for fuzzyband in cydia and it will tell if you can dowgrade the baseband. If you coud downgrade unlock by blackra1n or redsn0w ....ok good luck