After the iOS 4.2 jailbreak for iPhone 3GS old bootrom devices, now you can jailbreak and unlock iPhone 3G iOS 4.1 using unofficial PwnageTool Bundle released by the msft_guy. Process is almost similar, you’ll first need to download resources then move on to patching PwnageTool & cooking custom IPSW with it and then finally loading custom IPSW into the iPhone. Here is the guide to help you jailbreak iOS 4.1 iPhone 3G:

Jailbreak iOS 4.1 on iPhone 3G

Download:

• PwnageTool 4.1
• PwnageTool Bundles for iPhone 3G
• Apple iTunes 10
• Apple iOS 4.1 firmware for iPhone 3G

Place all files in same folder and follow the step by step guide posted here to jailbreak iPhone 3G iOS 4.1.