If you want to have a 2G running 3.0 UNLOCKED (NOT Jailbreaked, you wont be able to run cydia or any app installed using cydia, or cracked .ipa) follow these instructions:
1 - Go to 2.0.2fw.
If you are in a fw below (ie 2.0), just update or restore to 2.0.2. If you are in an updated fw (ie 2.2.1) just put your iPhone in DFU mode and alt + RESTORE (option+RESTORE for mac)in iTunes and choose 2.0.2 fw, that you'll downgrade your phone without any errors.
2 - Activate with QuickPwn RC3 for windows (Some got it working with RC2, i got it with RC3). Also, i dont know if there is a QuickPwn for mac, that can be used in this case.
3 - Open itunes, and select Setup as a new iPhone.(Later if you want you can restore to your backup).
4 - Now UPGRADE to 3.0. Using alt+UPDATE (option+UPDATE in mac).
5 - Done, you have an iPhone 2G unlocked in 3.0. If you want to get your backup, just right+click on the iPhone in iTunes, and select Restore from Backup, and you are good to go.
Abras