The iPhone-Dev team has released a newer version of Ultrasn0w tool for iPhone 3G/3GS users running the latest iOS 4.2.1. The unlock will allow users to use their phones on different carriers using the same AT+XAPP command exploit, which Apple reportedly fixed previously.
The new Pwnage Tool and redsn0w 0.9.6 beta 5 will update the phone’s baseband firmware to 06.15, which is said to be used in the iPad and is compatible with both the iPhone variants. The ultrasn0w can then be downloaded through Cydia by adding the official ultrasn0w repo.
Although it seems luring, updating the baseband could have its own repercussions like no way to downgrade to an official iPhone 3G/3GS baseband, could void the warranty and can’t even restore to stock firmware (will need custom IPSWs).
However, if you’re still willing to dive in, full instructions and download links are available at the link below.