PwnageTool bundle for iPhone 3G running final version of iOS 4.1 has been released. Unlike iOS 4.2 bundle for iPhone 3GS, this one has fully working Cydia and it also supports hacktivation.
Since the current version of Redsn0w reportedly has issues (breaks phone app, Cydia doesn’t work) jailbreaking iPhone 3G on iOS 4.1 (final version), you can use this bundle with PwnageTool to jailbreak your iPhone 3G on iOS 4.1 while preserving your baseband so that you can unlock your phone later with Ultrasn0w.
Guide for patching PwnageTool with Pwnage bundle is similar to the iOS 4.2 guide posted here. You can follow this guide to patch PwnageTool 4.01 so that it can identify iOS 4.1 for iPhone 3G.
Once you are done with the jailbreak, you can follow our step by step guide posted here to unlock your iPhone on iOS 4.x, on older baseband using Ultrasn0w.