How to Jailbreak iPhone 3gs on iOS 4.2 or 4.2.1 old and new bootrom
1. Download greenpois0n here
2. Connect your device
3. Turn off your device while connected to your computer
4. Open greenpois0n
5. Click "Prepare to Jailbreak (DFU)"
6. Hold the power button (top button) for 3 seconds
7. Hold the Home button (bottom button) + the Power button for 10 seconds
8. Release the Power Button
9. Continue to hold the Home Button until the screen on the program is changed
10. Press "Jailbreak"
11. Codes will start rushing down on your device, no matter what happens and no matter what the codes say, don't panic and DO NOT DISCONNECT YOUR DEVICE!
12. When the process is complete go on your iPhone and find the application that says "Loader" (NOTE: if you do not have "Loader," DO NOT PANIC you can just go straight to your new "Cydia" icon
13. Once in "Loader" press on "Cydia" and then select "Install Cydia"
14. Now you should have the "Cydia" icon on your screen
15. And your done!