iOS 4.1 which has been Released two days ago and was jailbroken on iPhone 3G and iPod Touch 2G using Redsn0w, Now it's Jailbroken also on iPhone 3GS by the well known tool Sn0wbreeze. iH8sn0w today updated his tool to Sn0wbreeze V1.8 and enable you to Jailbreak iOS 4.1 on iPhone 3G/3GS and iPod Touch 2G (Non-MC Model)..
Note: Your device must be jailbroken on iOS 4 or iOS 4.0.1, Don't Ignore this or your Device will be damaged.
After Jailbreaking You will be able to Unlock using Ultrasn0w 0.93 as iOS 4.1 don't Upgrade the Baseband, So follow the guide posted here to Unlock your iPhone 3G / 3GS.
Download Sn0wbreeze V.18 for Windows
Download .Net Framework [Required for Sn0wbreeze to work]
Download iTunes 9.2
Download iOS 4.1 [Developers]