According to MuscleNerd, an updated PwnageTool release is on the cards that will jailbreak iOS 4.2 and will preserve the iPhone unlock with ultrasn0w 1.0. That’s one of the useful features of PwnageTool that it lets you upgrade the firmware without updating the Baseband. That’s what exactly MuscleNerd said:

we are also releasing new PT (primarily for those who need to update FW without updating BB, to keep unlock)

So, while all eyes are on SHAtter exploit and Chronic Dev Team’s Greenpois0n jailbreak, an updated PwnageTool for iOS 4.1 / 4.2 will be a real bonus for those iPhone users who rely on carrier unlock.