Apple just released iOS 4.0.1 for iPhone 4, 3GS and 3G. The baseband was expected to be upgraded but it remained same so this means you can unlock iOS 4.0.1 on iPhone 3G and 3GS (old bootrom).

First you need to jailbreak your iPhone, here are guides to jailbreak iPhone and iPod Touch running iOS 4.0.1

• How to jailbreak iOS 4.0.1 on iPhone 3G with Redsn0w
• Jailbreak iPhone 3GS iOS 4.0.1 with Pwnage Tool (Unofficial) [by iHackintosh]

After jailbreak, follow these steps to unlock.

Note: Before you go for the guide you must know

- I will not be responsible for any loss of your device while attempting this jailbreak, do it on your own risk

How to unlock

1. Run Cydia on your jailbroken iPhone.
2. Tap ‘manage’ at the bottom of the screen and then ‘click Sources’

   

3. Tap the ‘edit’ button at the top right of the screen and then tap ‘Add’ button on the top left of screen.
4. Enter http://repo666.ultrasn0w.com as the source of URL and tap ‘Add Source’ button.

   

5. When your done with adding source, tap ‘Return to Cydia’ button and press the ‘Done’ button at the top right of the screen.
6. Select http://repo666.ultrasn0w.com/ from the list of user entered sources and select ultrasn0w from the list of Packages.
7. Tap ‘Install’ button at the top right of screen and then tap ‘Confirm’ to start the installation.

   

8. When installations completes successfully, restart your iPhone.
9. After reboot, hold the ‘home’ button and power off the iPhone and then turn it on again while you hold the ‘home’ button.
10. Done, you can now enjoy your unlocked iPhone.