Let me clear this upfront. This is strictly for iPhone 3G and iPod touch 2G (non-MC model) only. If your iPhone 3G relies on an unlock, DO NOT update to iOS 4.1 Beta now as the current version of ultrasn0w cannot unlock this new firmware.
If you have iPhone 3G (which doesn’t rely on an unlock), or an iPod touch 2G (non-MC model), then simply follow the guide posted to below jailbreak your device on iOS 4.1 Beta using Redsn0w.