In just a few short days the iPhone Dev-Team has worked out a fix for users interested in upgrading to 2.2.1 firmware without breaking the yellowsn0w unlock. As was announced earlier this week, the new firmware update from Apple changes the baseband version from 02.28.00 to 02.30.03, rendering yellowsn0w impotent.
If your device is an iPhone 3G and you're interested in unlocking now or in the future, you must follow the Dev-Team's Golden Rule. First and foremost do not use the iTunes update process without using PwnageTool. This means the official ipsw file for 2.2.1 must be modified so that the older baseband is preserved. Furthermore, the Dev-Team says this cannot be done with QuickPwn.
Do not use QuickPwn! The result of PwnageTool will be a custom ipsw file that updates the firmware to 2.2.1 without changing the baseband from 02.28.00. The older baseband version can still be unlocked by yellowsn0w on a jailbroken device.
The unlock is available via Cydia or Installer and has been updated to yellowsn0w 0.9.7.