According to Vincet, admin of iPhone Wiki, Ultrasn0w unlock for iPhone 4 02.10.04/03.10.01 baseband is not easy to crack and it might take a while before iPhone 4 users will be able to unlock their devices. For those who don't know, updating to iOS 4.1 on iPhone 4 upgrades its baseband to 02.10.04 and updating iOS 4.2.1 upgrades its baseband to 03.10.01.
[Update via @veeence]
I really had no idea what ThreadX is but after Googling a bit, i found that ThreadX is a Real-Time Operating System (RTOS) that uses various threads to run the various modules and operations. Like most of RTOSes, ThreadX uses a multitasking kernel with advance scheduling, fast interrupt response and very good memory management. Definitely the kernel of such OSes are hard to crash and thats why Apple used ThreadX to run the baseband of iPhone 4.
So if you are planning to update and jailbreak iOS 4.1 or iOS 4.2.1 on your iPhone 4, you should always use PwnageTool or Sn0wbreeze tool instead of jailbreaking through Redsn0w tool. PwnageTool and Sn0wbreeze helps you to preserve the baseband version of your iPhone allowing you to unlock the device by using the latest version of Ultrasn0w unlock tool.
Update#1: Unlock for iPhone 4 02.10.04, 03.10.01 & 04.10.01 Baseband is Almost Ready!
Update#2: Unlock for iPhone 4 02.10.04, 03.10.01 Baseband Delayed :(
Update#3: iPhone 4 02.10.04, 03.10.01 Baseband Unlock is SIM Dependent
Update#4: iPhone 4 Unlock is Possible Through NCK Brute Force Method