The iPhone Dev-Team Members MuscleNerd and Planetbeing has confirmed that even though the unlock for iPhone 4 with ultrasn0w is ready and all, they will still do thorough testing in the next 48 hours to make sure everything is working before it is released to public. This is what they said on their respective Twitter accounts:
Planetbeing: We will be releasing the unlock in 48 hours (after we conduct a private beta).
MuscleNerd: iPhone4 carrier unlock will be pushed after small test period (only @planetbeing and I have been able to test til now!)
Stay tuned as we will let you know whenever it is available, along with full how-to guide on unlocking the iPhone 4.