Please note however that Yellow Snow works with latest iPhone 3G firmware (02.28.00). If you only have a model not later than 2.2, just check the iTunes first before unlocking.
Furthermore, you will need to jailbreak your iPhone before following the procedure in Yellow Snow.
Very important note: There is an initial report that close to half of the unlockers where not able to make it. So I guess you have to be more careful in unlocking your iPhones.