I forgot to mention in my guide on how to unlock iPhone 3G that the version of yellowsn0w was upgraded yesterday itself, and many people who did it earlier and failed should try this new version of yellowsn0w 0.9.5.

This has some good fixes like STK issues and so, trying out the process with this newer version of yellowsn0w would be recommended. Some good things about this tool -

• You can use it now, and remove it through the app list, or through iTunes if you don’t want it for later.
• Its a very small 6kb program, so not going to play much with your RAM.
• It can be added through the sources list and you don’t need iTunes account or connection with iTunes for its download or installation.
• Many report that this tool if did not work, won’t make any other changes but just keeps silent.

Its a worth trying tool after you jailbreak your iPhone with firmware 2.2, and follow the steps as in the iPhone 3G unlocking tutorial.
Issues about the previous version, and fixes are discussed here.