MuscleNerd, one of the iPhone Dev Team memebrs has just tweeted that they updated Ultrasn0w to new version that can Unlock iOS 4 but this is not the waited unlock tool from Dev Team, as it works only 04.26.08 baseband. but the much waited tool will work on 05.12.01 baseband.

Note: A new ultrasn0w capable of unlocking 4.0’s baseband isn’t yet released because both the final 4.0 and iPhone 4 are coming out next week. Please be patient.

Steps:
Step 1: Check your baseband, Simply Go to Settings --> General --> About --> Modem Firmware and you will find your baseband there.
Step 2: If you’re running baseband 04.26.08 on 4.0GM, then you can Install Ultrasn0w by Going to Cydia --> Manage --> Sources.

Step 3: Now add "http://ios4.ultrasn0w.org" then hit Enter. Ultrasn0w will be added to your Cydia Source lists.
Step 4: After you install Ultrasn0w in your Cydia sources list, Search for "ultrasn0w 0.92" and install this app, and it will automatically unlock your iPhone 3GS.