Here is a good news for iPhone unlockers that Ultrasn0w is fully working on iOS 4.2.1 and iOS 4.1. MuscleNerd, team member of iPhone dev team just posted a tweet on his account that he has got Ultrasnow fully working on iOS 4.2.1 and iOS 4.1.
@MuscleNerd: Finally got ultrasn0w working fully on 4.2.1. Apple inadvertently(!) broke ultrasn0w via aggressive compiler optimization
Play stay tuned to this site. We will post the latest news about ultrasnow 4.2.1 release soon.