Dev-team's MuscleNerd tweeted today that he has managed to get Ultrasn0w working on iOS 4.2.1 and on iOS 4.1 with baseband 0.1.59.00 on the iPhone 4 and 05.15.04 on iPhone 3GS and iPhone 3G.
Below is his tweet:
We heard a progress update on the unlock a few days ago, when MuscleNerd initially announced the release date.
However, it should be noted that those who updated to 4.1 or 4.2.1 without preserving the baseband will have to hold off for now, because this version of Ultrasn0w doesn't support baseband 02.10.04.
Either way, the unlock should be releasing soon. We will keep an eye out.