Ultrasn0w is simply a replacement of yellowsn0w and it works on iPhone 3.0 OS. Ultrasn0w doesn’t matter the version of bootloader you’re using. As long as you’ve upgraded to iPhone OS 3.0 with baseband 4.26.08. It will work for you. Before you proceed with step-by-step instruction you may read the ultra tips from iPhone Dev Team regarding above matter here and here.

Note for T-mobile USA user:
Please ensure you disable 3G service before installing ultrasn0w. To use Ultrasn0w to unlock iPhone 3GS on firmware 3.0, please follow these procedures:

Step 1: Upgrade your iTunes to iTunes 8.2 version and backup your iPhone data by synchronizing it with iTunes. Upgrade your iPhone 3GS to iPhone OS 3.0 (or what so called firmware 3.0) using iTunes 8.2.
Step 2: Use PwnageTool or redsn0w (both Windows or Mac User) to jailbreak your iPhone 3GS
Step 3: Launch Cydia on iPhone and add “http://repo666.ultrasn0w.com” as the new repository URL. You can simply tap “Manage” -> “Sources” -> “Edit” -> “Add” to add the repository URL. If your not clear about this, checkout my post here on how to add sources to Cydia.

Step 4: Then tap “Search” and search for “ultrasn0w” application
Step 5: Install “ultrasn0w”

Step 6: Reboot your iPhone 3GS after installing ultrasn0w and enjoy!

You might be checkout the demo video from iPhone Dev Team on how to unlock iPhone 3GS using Ultrasn0w