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Hi there, I'm still trying to find a solution for the ultrasn0w unlocked iPhone 3g that I upgraded to 4.1 hence losing the jb and apparently locking the baseband to prevent further hacks.

I have just spotted this:
http://www.technorotic.com/2010/09/how-to-jailbreak-iphone-3g-3gs-having-ios-4-1-with-pwnage-tool/

Has anyone tried it yet? Would it work for already upgraded phones, too?

Since you have the new baseband, there is no unlock tool. Ultrasn0w won't unlock it.
Pwnage Tool hasn't been updated to work with 4.1 firmware and using this will likely screw up your phone. Even if it did work, you cannot unlock it.
 
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