This news is big and it comes straight from MuscleNerd - the member of the iPhone dev team who has been publishing all progress on the iPhone jailbreak and unlock scene on his twitter account.
Earlier it was released soon after the release of the iPhone 4 that as this phone contains a new baseband OS which is much different to that on iPhone 3GS or earlier generations, therefore Ultrasn0w would require a complete rewrite but the unlock is definitely possible.
The Dev Team and others have been checking through the iPhone 4, and have noticed the baseband runs a completely different OS than on previous models.
In a couple of tweets recently, MuscleNerd posted some of the progress of the iPhone 4 unlock with @planetbeing with pictures.
Good progress on iPhone4 unlock with @planetbeing: http://is.gd/dh2WE <-injected cmd! (Akin to http://is.gd/dh2Ze on 3G)
Next step is to keep the task backgrounded like we did for 3G/3GS http://is.gd/dh7Jh. Backgrounded task is the unlock.
This means that everything is ready and in shape and as soon as Apple pushes out the next firmware fix (which is due in a week or two), the dev team will release the jailbreak/unlock bomb for the iPhone 4 community.
[via RedmondPie]
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