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Updating to 4.1 will also update your baseband which currently is not unlockable. Like the previous poster said, use pwnagetool to create your custom 4.1 ispw which preserves your baseband so you can then unlock it.
 
yes there is snowbreeze 2.1 windows only.

I never recommend sn0wbreeze!

TS what you can do is search on the internet (or a friend w/ a mac) for a custom fw of 4.1 with the exact specifications that you want regarding BB and activation/hacktivation and then download redsn0w to use the custom fw and install the custom fw on your iPhone from your PC.
 
I never recommend sn0wbreeze!

TS what you can do is search on the internet (or a friend w/ a mac) for a custom fw of 4.1 with the exact specifications that you want regarding BB and activation/hacktivation and then download redsn0w to use the custom fw and install the custom fw on your iPhone from your PC.

Personally I wouldn't recommend this, given that someone could potentially package malicious software with the ipsw.
 
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