The dev team is still releasing updates for their 3g unlock. It is important that you update regularly if YellowSn0w is not working great for you. They also released some hints on their web site: 1) If you are on tmobile USA you should disable 3g in settings or sbsettings. 2) YellowSn0w does not work [...]

The iPhone dev team’s 3g unlock, yellowsnow is out. There are some changes in how it works. The 3g unlock will require you to be on baseband 2.28.00 which comes with iPhone OS v2.2. This means that those of you that pwned your iPhone to 2.2 without a full restore will now need to restore [...]