Last day, the updated iPhone firmware version  3.1.2 was released and can be updated through iTunes. Do you want to jailbreak it? Then here is a guide to do it. Just follow the steps given below.
1. Update your iPhone OS firmware to 3.1.2 using iTunes.
2. Download the IPSW files from the following links.
4. Follow this guide to jailbreak your iPhone 3G/3GS 3.1.2 firmware.