From unlocking perspective the important thing in iOS 4.0.1 released for iPhones (iphone 4, iPhone 3G and iPhone 3GS) is there is no baseband change and you can unlock you iDevice with ultrasnow. However, currently we can unlock only iPhone 3G because there is no jailbreak software available for iPhone 4 and the jailbreak tool for iPhone 3GS is still under development to support the firmware iOS 4.0.1.
Here is the Step by Step Guide to Jailbreak iPhone 3G on iOS 4.0.1 using Ultrasnow:
Step 2: After jailbreak, run Cydia, tap the manage button on the bottom, tap on Sources, click on Edit button of top and then tap Add.
Step 3: Now type this repo address in the sources: http://repo666.ultrasn0w.com (It is zero in ultrasn0w and not O).
Step 4: Tap on Add Source and wait for it to update.
Step 5: Now search for ultrasn0w via search field, tap it and press Install at the top right.
Step 6: Reboot your iPhone 3G and use any carrier on iOS 4.0.1.
Disclaimer: This guide is for testing & educational purposes only. I’m not responsible for any loss of important data or malfunctioning of your iPhone.