If you are proud owner of those rare iPhone 3G models which has 05.08 bootloader then you can get unlocked iPhone 3G running 4.0 OS. Yes you can get your iPhone OS 4.0 unlocked now. Recently released Redsn0w 0.9.5 can now jailbreak iPhone 3G 4.0. Once you have jailbreaked your iPhone 3G 4.0 you can install Cydia repository and access apps for jailbroken iPhone.
Fuzzyband Downgrader is the app of Cydia is now updated to support iPhone OS 4.0 Baseband 05.13.03 downgrade provided you have iPhone 3G with 05.08 bootloader. The trick is to downgrade baseband of your iPhone 3G 4.0 to unloackable version like 04.26.08 which can be unlocked using Ultrasn0w 0.92.
Below is the step by step guide to unlock iPhone 3G 4.0 using Ultrasn0w 0.92
Unlock iPhone 3G 4.0 with Ultrasn0w 0.92
1. Jailbreak iPhone 4.0 using Redsn0w 0.9.5, we have posted guide here.
2. Now follow our guide to downgrade iPhone 4.0 baseband using Fuzzyband Downgrader
3. Once you have jailbreaked & downgraded iPhone 3G 4 OS go to Cydia, Manage tab at the bottom.
Note: T-Mobile users should disable 3G before installing ultrasn0w.
4. Click on “Edit” and then on “Add”. You will be prompted to enter a url source. Type “http://repo666.ultrasn0w.com” and click on “Add Source”.
5. After successfully adding repository search for ultrasn0w in cydia and install the application
6. Restart your iPhone.
You should have unlocked iPhone 4.0 if you have followed above steps properly
Please Note: Above guide is education purpose only, we will not be responsible if you brick your iPhone by following above guide. Do it at your own risk.